The Catch drill isolates the leg action at the beginning of the drive phase.
Practice shoulder relaxation and leg-back co-ordination by moving back and forth between the Catch position and the beginning of the drive.
Concentrate on feeling the connection of the paddle in the water as it "locks on", with the arms and shoulders remaining in the same position while the legs begin the drive.
When practicing this drill:
There is no movement in the arms and upper body, all the movement is in the legs (a small backwards movement of the seat)
Keep a strong postural position
Relax your grip and hang off the handle with straight arms
Keep head up and eyes forward
Practice this drill for a few strokes then continue with steady state rowing, for example for practice the Catch Drill for 5 strokes every minute and row for the rest of the minute.