Lets assume we have a TF/VP Snap clone setup in cascaded A (Source) - B (Target Clone) - C (Snapshot) format. If we start a clone session between A and B with -precopy option what exactly happens in background? and the what happens if we "Activate" it?

Lets assume we have a TF/VP Snap clone setup in cascaded A (Source) - B (Target Clone) - C (Snapshot) format. If we start a clone session between A and B with -precopy option what exactly happens in background? and the what happens if we "Activate" it? Precopy means just copy everything block wise from source to target?Please elaborate in details what happens exactly.