Lag vs casting and the power that is delivered to the ball can be explained simply as when that power is used. Is it early in the swing or is it at impact? The longer we can store the energy in our swing (lagging) and the closer we can release it at impact, the more efficient our swing will be in relation to the power we generate.
For example, there are two people who swing the club at 100MPH but one golfer applies the 100MPH early in their swing (casting) while the other delivers the 100MPH at impact (lagging). The golfer who releases the 100MPH of energy at impact will inevitably transfer more energy into the ball and the ball will travel further.