We have covered momentum before but it could be the most important concept for all swimmers to understand.
Do your best to convey to all swimmers the importance of maintaining and creating speed. This takes focus and efficiency!
Maintaining momentum from a start, turn, the finishing of strokes and kicking are extremely important.
Teach your swimmers not to slow down underwater before swimming on top. The underwater part of swimming off the start and turn is where a swimmer can be at their fastest. They must be efficient in their break-outs, streamline, underwater pullouts and of course fly kick. Why do you think we have the 15-meter rule in swimming? Because the underwater part of swimming can be the fastest part of the race. Plan practices accordingly and stress its importance.
I feel the next big momentum killer is the timing of the breath in all strokes. Teach the swimmers when to breath in each stroke. If the breath is taken at the wrong time the mechanics of the stroke will slow and will usually be accompanied with a slower kick. The timing of all strokes/kicking and breathing is covered in depth in the Faster Swimming book.
Judging the walls for turns is another biggie. If you can get the swimmers to understand that if they slow down into their turns it will affect the speed of the turn and the speed off the wall severely slowing momentum. Time their turns in practice and meets to get your point across. The 14 week and 23-week training books dictate when to time turns and plans for it weekly.