Let's say you are sculling. Probably rowing 24-32 SPM. At 30 SPM, and assuming an even ratio of recovery to drive, you are spending one second on the drive each stroke. It seems to go by too fast to be thinking of breaking down the motion; just pull, right?
At full reach, the shafts of your sculls are approaching a position parallel to the hull. The first part of that second on the drive is simply connecting with the water and getting the blades moving. Assuming you start with arms straight and do not lift the shoulders and back to begin, you are beginning the motion of the drive and setting the blades with the legs. At full reach, the knees are compressed. This initial motion allows the legs to transition from recovery to drive without delay and begins to open the knees to a position of greater power than you have in this compressed position.
Equally interesting, as the drive progresses, the sculls move through the beginning of the arc to the portion closer to perpendicular to the boat. In the arc of the blade 'through the water' from before perpendicular to after - call it the middle 75% of the drive, the power you put into the oars is most useful at moving the boat forward; the sculls drive mostly toward the stern.
With the legs in a stronger position during this portion of the drive and the blades in a more effective position, powerful acceleration is called for. And moving the back (hips, back and shoulders) through the range of motion they will apply on the drive at this time compounds the effectiveness of the legs.
Then the legs are down and feet pressing against the foot stretcher, the shoulders back and the elbows pulling through while the fingers and thumbs tweak the handles as the blades lift and feather and begin a return on the recovery.
See http://www.last500.com/ for more.