Constraints led approach (CLA) drill to improve skills ant tactics in standard situations. Using different balls (from red to yellow). You can also mix old and new balls.
With changing constraints by using different balls, different court sizes, etc. the player acquires indivual solutions for changing tasks. The CLA improves technique und creativity in always changing challenges. This corresponds to the real situations in the game and is nearer to a creative „learning in the street“ approach than the constant repetition of the same situation how we know it from methodical traditionalism.
One often unattended aspect is the position of the coach. Changing positions from where you bring the ball in the game, varying the spin, the distance of the first ball to the player improves learning and skills. http://www.tms-tennis.de/inner-coaching/coach-where-are-thou/