TO MAKE A GOOD slack-leader cast, I don’t advocate casting with high line speed and stopping the rod sharply to create a shock in order to put slack in the leader. When you shock the rod, it’s often hard to tell where the fly is going to go, and you are actually pulling some of the curves out of your line.
I prefer to make a controlled soft cast with no shock. Begin with an elevated forward cast, apply power softly, and after the loop is formed, drop the rod tip to the surface of the water. This gentle cast is accurate and puts lots of slack in your leader and line. If a high-speed cast is made, the line recoils at the end, jumps back, and ruins the delivery. If you aim the forward cast high enough, this becomes a stack cast. The principles are the same, though you can make this cast without a low backcast.
The key is to stop and drop your rod tip gently to the water. The rod must be lowered immediately after the speed-up-and-stop. If not, the line held aloft by the rod tip will sag back, pulling the waves from the line.