Useful-skew clock ECO

Useful-skew clock ECO. A flip-flop sits between two logic stages: one stage feeds into it, the other leaves it. When the stage feeding it is setup-critical but the stage leaving it has slack to spare, you can retard that flop's clock: a later capture edge lends time to the tight stage, at the cost of the loose one, which now launches later. This is useful skew. The data path is untouched, only the clock arrival at one flop moves, transferring slack from where it is plentiful to where it is scarce.

Skew too far and the loose stage goes negative in turn, so there is a window.

Think of it as one fixed pool of time split between the two stages, with the shared clock edge deciding the split. Click a stage to lend its timing margin to the other: click stage B to move time into the tight stage A, or click stage A to move it back to stage B. Under the hood each click retards or advances FF2's clock, shown on its clock branch. Land both stages at or above zero, and prefer the balanced point (equal margin on each side, the safest against timing spread). Then Submit skew.