Hi,
One way to design flops is to use pass-gates to sample the D-input signal.
The pass-gate is controlled by clk and clkb.
You do not want the signal that is going through the pass-gate to be changing when the clock turns the pass-gate off.
If the signal is changing while the pass-gate is switching, what value is being passed to the MASTER latch? Who knows...could be 1, 0, or mid-level (bad).
So, to ensure this never happens timing checks are put in the simulation and sta models.