With the enthralling second Ashes Test at Lord's concluding with a nerve-shredding stalemate, attention is quickly turning to the third Test at Headingley, set to start in just four days' time.

All the focus will be on the health of Steve Smith and his recovery from the brutal blow inflicted by England quick Jofra Archer on the fourth day.

Australia's talismanic batting stalwart took no part in the dramatic conclusion at Lord's due to an overnight "deterioration" of his concussion symptoms.

With such a short turnaround between Tests, Smith faces a race against time to prove his fitness and take his place in the side at Headingley.

What happened?

In case you missed it, during Australia's first innings — with England defending a first-innings score of 258 — Australia had recovered from a patchy start to pass 200 for the loss of six wickets, with the unflappable Smith cruising towards a third-straight century of the series.

However, Archer had other ideas.

The Barbados-born quick hit speeds of 154 kilometres per hour during a furious spell of fast bowling which completely changed the complexion of the day's play.

Shortly after sending a blistering delivery into Smith's unprotected forearm, Archer hit Smith on the neck, just below the left ear with a 148kph thunderbolt that sent the 30-year-old former captain sprawling headfirst into the pitch.

Amidst an eerie hush around Lord's, Smith underwent a series of checks on the field before retiring not out on the advice of team medical staff to undergo further tests in the dressing room, as per Cricket Australia concussion protocols.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 2 minutes 30 seconds 2 m 30 s Steve Smith copped two big blows from Jofra Archer (Pic: Reuters)

What is a concussion test?

The Cricket Australia Concussion Protocol says if a player receives a blow to the head or neck, a concussion assessment should be carried out.

Initially this is an on-field assessment to determine whether a concussion is established or suspected, such as whether the player feels dizzy or has a headache or nausea, or has suffered a loss of consciousness or is unsteady.

The Cricket Australia policy allows for replays to be checked by medical staff to see if there are any obvious signs of concussion.

The policy also makes clear the game situation makes no difference, stating "it is not relevant to the operation of this Policy, or the assessment of the Participant by the medical staff member or contractor, that the Participant is in a last wicket partnership to save or win a match".

If further assessment is required, the player has to be removed from the field for further tests, either through a SCAT5 or CogSport test.

Steve Smith's injury was harrowing in the context of Phillip Hughes's death. ( AP: Alastair Grant )

So what do those tests entail?

A SCAT5, or Sport Concussion Assessment Tool, fifth edition, is "a standardised tool for evaluating concussions" that takes into account a variety of short tests that take at least 10 minutes to complete.

The test is used by sports groups around the world, including FIFA, the Olympics and World Rugby to name a few.

The immediate assessment can take place on the field, but a full test must take place in a quiet environment such as the dressing room.

The tests include:

Immediate memory — such as repeating a series of five to 10 random words back to the tester

Immediate memory — such as repeating a series of five to 10 random words back to the tester Concentration — such as reciting the months in reverse order

Concentration — such as reciting the months in reverse order Delayed recall — remembering those previous words at the end of the test

Delayed recall — remembering those previous words at the end of the test Balance — standing on one leg

Balance — standing on one leg Tandem gait — walking in a straight line using a heel-to-toe motion

Tandem gait — walking in a straight line using a heel-to-toe motion A finger-to-nose test

CogSport is a computer program that uses "computer-based neuropsychological tests designed … to detect mild cognitive changes in concussed athletes".

The program, which is used in the AFL, takes 15 minutes to run and compares a baseline assessment from the off-season to the tests after the injury.

Why did Smith continue to bat after being hit?

Smith was hit twice by Jofra Archer on Saturday. ( Reuters: Paul Childs )

Having passed the initial concussion tests on Saturday, Smith was free to play.

About 40 minutes after leaving the field Smith returned, making 92 before being uncharacteristically trapped lbw by Chris Woakes in a performance that appeared slightly disjointed.

Australia coach Justin Langer explained at the end of day four that Smith was desperate to continue his innings.

"He went through the concussion protocols and seemed to be coming up OK," Langer said.

"He said 'I can't get on the honours board unless I'm out batting'. All he was worried about was that he wasn't going to be able to play his forward defence because his top hand grip was hurting.

"I asked him over and over, privately two or three times and in front of the group, the medical team cleared him and he said he was ready to go."

So why didn't Smith play on Sunday?

On Sunday morning, Smith reportedly woke up with "a bit of a headache and a feeling of grogginess".

Following Cricket Australia guidelines, Smith underwent more testing, which showed "some deterioration", meaning Smith was withdrawn from the Test by team doctor Richard Saw.

Australia then successfully applied to use a concussion substitute.

ICC Rules regarding concussion substitutes 1.2.7 — If a player sustains a concussion or suspected concussion as a result of a head or neck injury during the course of a match, a Concussion Replacement may be permitted. 1.2.8 — The ICC Match Referee should ordinarily approve a Concussion Replacement Request if the replacement is a like-for-like player whose inclusion will not excessively advantage his team for the remainder of the match. For example, a specialist batsman can replaced a batsman, a bowler for a bowler, etc. If an all rounder is selected to replace a batsman, restrictions may be placed on whether that player is allowed to bowl. 1.2.10 — Once the Concussion Replacement has been approved by the ICC Match Referee, the replaced player shall play no further part in the match.

Cricket Australia have been leaders in this area, introducing concussion substitutes in the Sheffield Shield in 2017.

Cricket Australia said in its statement "statistics show that 30 per cent of concussions in Australian cricket are delayed".

"It is not uncommon for players to pass their tests and feel well on the day of an injury and then display symptoms 24-48 hours later."

Smith was accepting of the decision when he spoke before play on Sunday.

"With the tests that I've done and how I've woken up today, I think it's the right decision," Smith said.

"I'd obviously love to be out there trying to keep performing and try and help Australia win another Test match, but I think the right decision has been made."

Steve Smith has been in sensational form on his return to the Test arena. ( AP: Alastair Grant )

How long does Smith have?

Not very.

The third Test starts at Yorkshire's Headingley ground on Thursday.

"I'll obviously be monitored very closely over the next few days," Smith said prior to play on Sunday, after being ruled out.

"With a pretty quick turnaround in between Test matches, I'm hopeful that I can make a recovery and be OK for that."

The Cricket Australia policy states "a return to staged physical activity must not occur for at least 24 hours after a concussion diagnosis".

If Smith shows no further signs of concussion, he should be able to line up with the rest of the team in Leeds.

However, if there is any regression or return of his symptoms, he may have to wait and could miss out.

Regardless, Smith cannot return to play unless a medical officer gives clearance and that will only happen he passes the SCAT5 and CogSport tests.

Smith is the leading run scorer in the series so far with 378, averaging 126.00. He has two 100s and a 50 to his name from just three innings.

Therefore, it is no surprise captain Tim Paine is hopeful his former skipper will be able to take his place in Leeds.

"Marnus [Labuschagne] came in and batted as well as anyone did in the Test match, so we've got him up our sleeves," Paine told the BBC.

"We've [also] got Marcus Harris who is ready to go, but I reckon it will be pretty hard to stop Smithy walking out next week."

Cricket Australia's statement was less clear about Smith's prospects.

"In terms of Steve's availability for the third Test, this will be considered over the coming days, but the short turnaround to the next Test is not in his favour," the statement said.

"Steve's fitness will be assessed on an ongoing basis."