The Push-press is a variation of the strict Press in that the athlete generates power from the legs initially, then transfers that power through the core, up to the upper extremities, which helps press the bar overhead.  They key is using a quick, short, dip that takes advantage of the stretch reflex in the legs.  Slowly dipping down or holding bent knees at the bottom will be counter productive and not produce as much power.  Watch this video for some more detailed instruction, paying specific attention to the straightness of the back, not leaning forward or backward during the dip.