The 4013 contains two independent D-type flip-flops with asynchronous set/reset inputs. Whenever the set or reset pins go high, the appropriate output is expressed immediately on the outputs. When set and reset are low, the output shows the data at the input at the time of the last low-to-high clock transition. This is then held until the next transition.
For more information on D-type flip-flops, refer to this module.
