In swimming, a critical component of a technically sound and efficient freestyle swim stroke is the ability to kick effectively while simultaneously engaging the core appropriately. An effective and efficient kick combined with a tight core allows you to maintain a streamlined body position (horizontal body position in the water with minimal wiggle from side-to-side) while propelling yourself forward with the kick (albeit slightly as the kick shouldn't contribute massively towards propulsion in swimming, particularly long-distance swimming). The vertical kick is a challenging, but great drill for developing the ability to kick appropriately while learning to engage the core in the water. Click the video above to watch and learn more about this drill. This drill can be done with and without fins. As you'll learn in the video, I would begin this drill with fins if you are new to it and progress to not wearing fins over time. You can incorporate this drill as part of your warm-up or as part of your main set as this drill involved quite a surprising increase in heart rate due to its difficulty.
-Ryan Eckert, MS, CSCS