This is when we have taught our players to use the CU.
Use it immediately after a batter fouls a ball straight back. It is at this point the batter feels his timing is just right for th efastball and will most likely be wanting to see another fastball. Throwing the CU will almost always draw a swing at this point.
Use it for the first pitch. Most batters are looking for timing the fastball and when they see the CU they usually won't swing and wait for the next pitch to be the fastball- even if it is finishes right down the heart of the plate.