To really sprint in freestyle, you’re going to have to develop a strong kick. Focusing on how much you use your legs can really help this.
Why do it:
Learning to drive your stroke from your kick is one option to really swimming fast. Because many swimmers focus so much on the pull, switching your focus during sprints to the kick can help you develop a new tool.
How to do it:
1) Start with a length or two of freestyle using very little kick.
2) Just as with any descending set, as the lengths go by, start to increase the intensity of the legs.
3) Continue to drive the stroke from the legs and allow the arms to match whatever rhythm that develops.
4) On the fastest swim, rather than focusing on the arms, shift your attention to over-kicking to drive you forward.
How to do it really well (the fine points):
This is really a mental drill as much as a physical drill. Pinpointing all your attention to your legs during fast swimming will help you increase the effectiveness of your kick. If you spend most of your time thinking about your pull, switch the focus to the legs every once in a while to make sure you’re leaving nothing to chance.