20 x 50 @ 1:00 (mostly), all at 400 IM pace
Start with fly, and keep doing fly until coach announces at a time of his/her choosing, that this will be the last one fly. Swimmers do that repeat fly @:40, and then immediately do the next one backstroke. So the stroke transition happens at a time the athletes are not anticipating. Keep swimming backstroke until the coach decides to transition to breaststroke, etc.