Minecraft wiki entry for Memory, associated text: If the input J = 1 and the input K = 0, the output Q = 1.
When J = 0 and K = 1, the output Q = 0.
If both J and K are 0, then the JK flip-flop maintains its previous state.
If both are 1, the output will complement itself — i.e., if Q = 1 before the clock trigger, Q = 0 afterwards.