Two J-K flip-flops can be used to make a master slave J-K flip-flop (as shown above). Observe carefully how the clock is fed to both the J-K flip-flops. To the first flip-flop clock is fed directly, but to second inverted clock is fed.

Now, when clock is high, first flip-flop is enable and latches the output on Q' and Q'BAR. Clock = '1' for first flip-flop is clock = '0' for the second (inverted clock), this invalidates second flip-flop till clock= '1'. When the clock becomes 0, this invalidates first flip flop and enables second, now the value latched on first flip-flop's output will be available on second's flip-flop output (Q and Q BAR).

Input to the J-K master slave flip-flop appears at the output( Q and Q BAR) when clock changes from high to low, hence this flip-flop becomes -ve edge flipflop.

Similarly a +ve edge flip-flop can be made by feeding direct clock to second J-K flip-flop and inverted to the first one.