The Marine Corps push-up starts by dropping flat on the stomach and then pushing up into the starting position with a “Marine Corps” yell and then do double dips where they do two push-ups for every one rep counted.
The Navy, Army, and Air Force start by dropping directly into the starting position and then count each push-up as a rep.
For a push-up to count, the Marine Corps say the chest must briefly touch the floor. The Army and Air Force say the chest can touch, but never to rest or bounce to build momentum. In the Navy, if the chest touches the deck, the test is over.
The Army and Air Force say that if the backs bows or the waist sags, that rep is not counted and they may bow their backs only while resting in the up position. However, the Navy says that when the perfect push-up plane is broken, the test ends.