Race around condition is the most important condition in Digital electronics. In J-K Flip flop, when J=K=1 the output changes its state. When a clock pulse width tp is applied the output will change from 1 to 0 after a time interval of Δt, where Δt is the propagation delay through two NAND Gates in series. Now after Δt we have J=K=1, Q=0, After another interval of Δt, output Q will become 1. Hence the output will oscillates back and forth between 0 and 1 in the duration tp of the clock pulse width.