Greater forces are more tiring, so assume that a constant force all the way through the stroke is most effective and economical. Then as the boat accelerates, the force must be applied increasingly quickly (increasing power).
Also as the oars swing through the arc, in the middle of the stroke the forces are closer to the direction of travel, so cause more acceleration, and require more power.
The skill needed is to maintain a constant force, while exerting a variably increasing power through the stroke.
You can sense the force by the feelings of pressure in your fingers and the soles of your feet. These should be constant through the stroke.
To achieve this complex sequence, let your body and subconscious mind take control. Deliberately relax while keeping the sensations of force in the fingers and feet constant, and the sensation of muscular effort least at the beginning and greatest near the finish.