On an erg, extra force produces an immediate improvement in the split. In a boat, the blade has to transfer energy into the water while the body transfers force through the feet and the shell. Not every hard pull becomes useful propulsion.
There is also “check.” The rower’s body moves relative to the shell, causing the boat to accelerate and decelerate. Rushing the slide, stopping sharply at the catch, or throwing the upper body can disturb momentum. Large speed fluctuations waste energy because water resistance rises quickly with speed.
Crew boats add synchronization. If rowers do not apply pressure together, the same combined effort moves the boat less effectively. The goal is not to row softly. It is to connect before accelerating, apply pressure smoothly, move patiently on the recovery, and match the crew.