Before Jeter took the baton from Bianca Knight at the Penn Relays, for example, the runners had plotted out a day earlier the exact point on the track that Knight needed to reach before Jeter would take off into the fly zone, marking the spot with a piece of tape. Typically, Jeter said, two Olympic-caliber female runners will start by pacing off 25 steps back from the triangle; they then run a test pass to see if the movements match up smoothly.

If the pass happens too early in the zone, before the outgoing runner has a chance to reach top speed, the pair can move the piece of tape away from the fly zone and delay the exchange a few steps. If the pass happens too late, putting the outgoing runner perilously close to the end of the 20-meter zone, then the piece of tape is moved the other direction.

The spacing is critical because using a silent pass — as most teams do in a loud stadium — involves having the receiving runner put a hand back to accept the baton after a predetermined number of steps. If the incoming runner is not there (or gets there too soon), disaster ensues.

“You never get the magic number on the first try,” Jeter said. “And you want to do it as close to the race as possible. Doing it weeks early doesn’t help anyone because you don’t know how someone will be running weeks later.”

Where is the absolute best spot for the exchange to take place? Dr. Aki Salo, a senior lecturer in sport biomechanics at the University of Bath in England, said his studies had shown that teams should strive to make the handoff 15 meters into the 20-meter passing zone. Even later would be better, Salo said, because then the outgoing runner could reach a higher speed. But leaving five meters of room in case of an awkward pass is prudent.

A fluid exchange can make the difference between a successful race and disappointment. On a good pass, the baton spends about 1.8 seconds in the zone, said Salo, who has worked with Britain’s track team for more than a decade. A bad pass might have the baton there for 2.0 seconds. With three exchanges in each relay, poor passing can cost a team half a second or more — an eternity in a sport where finishes are often decided by hundredths of a second.

At the 2007 world championships, Salo videotaped the British relay team during the preliminary heats. After analyzing the film, he noticed that two of the exchanges came far too early in the 20-meter zone. After he consulted with the team’s coaches, they suggested moving the tape on the track.