The phrase "knee tucks" refers to a physical exercise where you bring your knees up towards your chest while keeping your back and feet on the ground. Full definition
f. Alternate these with single - knee tucks in a running motion, keeping your hips up and body straight at all times; don't let your back sag or drop your hips. (womenshealthandfitness.com.au)
The key to performing these is that you want to land and immediately spring back up off of the ground for the subsequent knee tuck jump. (brandonricheyfitness.com)
If the team is set to do core strength, back and freestylers might perform bicycle crunches and side planks while breastrokers and flyers might try pikes and knee tucks on a swiss ball. (blog.bridgeathletic.com)