The idea behind this is the necessity for you to master these exercises in order to set the stage for future gains in muscular size and strength. (fitnessandpower.com)
If you've mastered an exercise on flat ground, try throwing in an unstable surface and see how well you respond. (blog.bridgeathletic.com)
If this exercise is too challenging, first try mastering this exercise with both legs at the same time. (best-leg-exercises.com)