BOSTON – The Marcus Smart Experience is Game 5, waning moments of the conference semifinals clincher against the Philadelphia 76ers, and Smart missing a free throw he tried to make and making a free throw he tried to miss before swooping in like a free safety to make a game-saving steal to make up for all of it. It’s Game 1 of the conference finals, early fourth quarter, and the Cleveland Cavaliers are chipping into a double-digit lead. Smart bricks a driving layup, which could have given the Cavs momentum, but then, after a turnover, knocks down a 3-pointer that effectively puts them away.

The Marcus Smart Experience is a wild ride.

[Yahoo Fantasy Football leagues are open: Sign up now for free]

As the Boston Celtics march toward one of the more improbable Finals trips in recent history, Smart is the team’s wild card. Defensively, Smart is rock solid. If Al Horford is the connective tissue that binds together Boston’s top-ranked defense, Smart is the weapon coach Brad Stevens can deploy in it. He defends four positions, as comfortable chasing JJ Redick around screens as he is battling Tristan Thompson in the paint. With Smart on the floor, the Celtics’ defensive rating in the postseason is 98.9; when he steps off, it falls to 108.4.

Smart is the burly teenager who grew up dreaming of being a power forward but only sprouted to 6-foot-3. He doesn’t just like physical play — he craves it. Most guards get help when they are switched on to bigger players. Smart routinely rejects it. In the final minutes of Game 5 against Philadelphia, Smart was overwhelmed in the paint by Dario Saric, the Sixers’ 6-10, 223-pound forward who flipped in a layup to extend Philadelphia’s lead to four. Two possessions later, Saric posted up Smart again. This time, Smart held his ground, forcing a Saric turnover that preserved a tied game.

Marcus Smart drives against the Cavs’ Kyle Korver during the second half of Game 2 Tuesday night. (AP) More

“He got me once, and it was a good play,” Smart said. “He wasn’t going to get me again.”

And not asking for help?

“I pride myself so much on defense, holding my ground,” Smart told Yahoo Sports. “I try to make it so if I can guard without help, it forces us to stay in our shell. That sends a message to another team when you have a guy who has been dominating and the other team tries to bring help and the guy they are helping tells them no, and then he gets the stop, that’s demoralizing to an opponent. When they feel they can’t beat a guy who doesn’t need help, that’s a problem.”

Offensively, Smart is more unpredictable. On his good days, he’s a power guard who can score through contact and be a threat from beyond the 3-point line. In Game 2 against the Sixers, Smart racked up 19 points and made 40 percent of his threes. On his bad days, he’s erratic, trying to score through multiple defenders and missing badly from beyond the arc. In Game 3 of the conference semifinals, Smart tallied just nine points, missing 6-of-7 threes.

Stevens’ message to Smart? “Let it fly,” Stevens said. Stevens’ playing career topped out at Division III DePauw, but his coaching style has been significantly shaped by his experiences as a player. He doesn’t like to pull guys with early foul trouble because he remembers how it used to affect his rhythm. And he hates publicly talking about his players’ shot selection because he knows how it would impact him.

“When I was playing, I was pretty weak mentally, and I think that one of the reasons why I’m hesitant [to talk shot selection] is because it would really affect me, in obviously a way lower level,” Stevens said. “But I think that I want guys playing free. I want them excited to make a mistake and learn from it and move on. You know, Marcus Smart makes a ton of winning plays, and because of that, he deserves to take those opportunities because we all believe he’s going to make them.”

Story continues