There are three state for the circuit:
1,0, and bi-stable state
when the clk is high, the self biasing circuit(regenerative circuit) strives out of the former 1 or 0 state, Vout+ and Vout- settle down to the same voltage( at least very closely voltage level).
when the clk is low, the branch of vin- or vin+ ( which one is low enough)will sink a current from the self biasing circuit, the vout+ or vout- node is pulled down, the self biasing circuit will funciton as a positive feedback circuit, make the low one more lower, make the high one more higher.
M5 and M6 are used to read the input signals.