The pre-cast is a simple motion that lifts some (or all) of the fly line off the water and gets the leader moving. It’s an elegant solution to a common problem.
When the dry fly drift is over, simply activate the line and get it moving before starting the backcast. The motion of the pre-cast pickup breaks the hold of surface tension. And that’s the key. Once the surface lets go of the line, it is easily lifted off the water with minimal disturbance . . .