Useful skew
Beginner
Making the clock tick reach one memory cell a bit late on purpose, to give a slow calculation more time.
Novice
Delaying the clock to one flip-flop on purpose so the slow calculation feeding it gets more time. The time comes from somewhere: the next calculation, which starts at that flip-flop, gets less, and that flip-flop’s hold check gets tighter.
Expert
Fishburn formalized it in 1990 as a linear program over every flip-flop’s clock arrival time. Production tools apply it step by step after CTS as concurrent clock and data (CCD) optimization, limited by hold checks and by loops of paths, around which the skews cancel.