The finish is among the least frequently practiced skills in our sport, in my observation. This drill serves as a mini-progression to help swimmers adjust to the presence of the wall appropriately during a finish, thus practicing for that gold medal moment.
We did this as a set of 16 x 50, 6 done like phase 1, 6 in phase 2, and the final 4 in phase 3. Here's the progression.
At the conclusion of the 50, the swimmer takes his last stroke at the backstroke flags and then positions his body for the finish, kicking strong all the way to an extended touch.
Same idea as the first phase, just move everything closer to the wall. Take the last stroke halfway between the flags and the wall. Make sure the swimmer is paying close attention to the spacing with the wall.
Now do an all-out finish, touching with the body at maximum length. The swimmer should touch with the fingertips. With the wall-judging ability honed in phases 1 and 2, the swimmer should be able to time his finish very precisely.
Try this progression for any stroke. You never know when your finish will make the difference between gold and silver! Thank you to Heath Hudgins (the swimmer in the videos) for being a willing example.