When running (that is normal forward movement without double-tap W) upon going prone the character will do a small seamless dive onto the ground and immediately continue crawling (in fact the animation looks a bit awkward and he skits a bit across the ground in the beginning). However, when sprinting (that is with double-tap W) instead of doing the same or a similar dive, the character will abruptly stop and then slowly go prone. He should probably do the same diving motion instead.