Such a feature would mimic race conditions and provide automatic and accurate timing for that segment...
To start a time trial, the watch would simulate a race start sequence - press the start button, hear the trill which would represent "Take your marks", and then a beep sounds approximately 2 seconds later which indicates GO and starts timing!
To finish, the timing would stop when the arm holding the watch stops abruptly as that hand hits the wall at the expected lap count; in other words, if a 100 had been entered in a 25 yard (meter) pool, the timing would stop at the end of the 4th lap.