|By allowing the resistance mechanism to move on the main bar, and in particular by reducing the mass of that mechanism to the mass of a single scull and then carefully fine-tuning the mass and resistance of the flywheel, and lastly by allowing Limited Instability of the seat, there are many significant benefits.
Firstly, the feel and resistance almost perfectly mimic the feel and therefore the rhythm of the single scull (or portion of a crew racing boat).
Secondly, there are major changes in the forces acting on the body.
Compared to a fixed head rowing machine, a 600% (yes, that is six hundred percent) reduction in the forces acting at the beginning and end of the stroke when the body changes direction; so the knees and the lower back don't receive the jerk that is so noticeable when rowing on a fixed rowing ergometer. Instead the user feels the smooth transfer of power right from the start through to the finish - a finish without the tug on the hip flexors so characteristic of other rowing machines (see article Fixed vs. Dynamic Ergos)
The coordination pattern is changed. As just one example, consider this: At the catch on a fixed head rowing machine, up to 90% of the rower's leg strength is needed just to stop his or her body from crashing into the front stops; it's just like a car braking heavily as Dr Ian Bernstein notes (link to Regatta article). So nearly all the leg strength has been used just to stop the body, and in the jerkiest of manners, before we even begin to take the stroke. In a lightweight boat or on a Rowperfect Indoor Sculler, the diametric opposite applies: you don't need more than 10% of your leg strength to reverse the body's (minimal) momentum, leaving all that extra power to apply smoothly through the drive phase of the stroke.
As another example, consider this: On a fixed-head machine the knee produces three times as much power as the hip joint. In a scull or on an Indoor Sculler, the hip produces three times as much power as the knee; the diametric opposite again. Think of the extra power that means you gain from the gluteal muscles, the source of power not just for rowing but for so many sports.
In a boat or on a Rowperfect Indoor sculler, the rower uses the hamstrings smoothly to draw the boat back to him during the recovery. This has a serious side-benefit: it all but eliminates the hamstring/quadriceps imbalance so characteristic of single-action machine usage, and so heavily implicated in lower back injuries.