Master-slave vs jk flipflop
A JK flip flop with the clock negated has the truth table for a normal JK flip flop, however with the clock negated, the outputs are shifted by half of a clock cycle. 

A master slave flip flop looks like this:




Source: Circuits Today

If you look at the truth tables for both, there isn't much difference. However looking at the gates of the negated clock input, Q can change at the same time as the clock and create a race condition. With the delay of the extra gates in the Master Slave JK flip flop this is not possible and prevents race conditions.