Confusion regarding PRESET and not(PRESET)
PRESET and CLEAR work like this:

If PRESET is LOW and CLEAR is HIGH, Q is HIGH regardless of what happens on J,K, and CLK.

IF PRESET is HIGH and CLEAR is LOW, Q is LOW regardless of what happens on J,K, and CLK.

IF PRESET is LOW and CLEAR is LOW, Q is undefined. (The two inputs are fighting each other.)

IF PRESET is HIGH and CLEAR is HIGH, the rules concerning the J,K and CLK inputs for a JK flip-flop apply.

In other words, if neither PRESET nor CLEAR is asserted, the flip-flop operates normally, However, asserting either will override the state of the flip-flop. And if both are asserted, the result is unknown.