Here is the issue of using the torso of the runner carrying the baton as that person crosses the mark that designates the distance:
4X100
The final exchange happens just as the baton reaches 100m to go.
There are three ways to catch this split, two using a runner's torso:
- You can take the split the moment the outgoing runner crosses the line - this would be the earliest to take the split and would make the 3rd leg faster and the 4th leg slower.
- You could take the split the moment the incoming runner crosses the line - this would be the latest to to take the split and would make the 3rd leg slower and the 4th leg faster.
- You could take the split as the baton crosses the line - this would give an in between split of the first two.
So if you use the torso - whose torso are you using if the exchange happens right at the line?
That's why you time the baton for the exchanges.
Of course the torso is used for the finish and the start goes with the gun.
Timing the torso or the baton gives the same result in most cases.
It's only an issue if teh exchange happens right at the line.