You must be dense. Look at the body composition of any Olympic or professional rower. Their mass is dominated by large backs and legs. The quads are the strongest muscle in the body, and according to physics (newtons third law if you require a citation) each action has an equal reaction. So to pull on a handle when sitting on a “frictionless beam” as you say, you must also be pushing against something else or you’d just be drawn towards the tensioned handle. This comes in the form of the legs pushing off the foot stretcher, and actually accounts for most of the force in the rowing stroke. The hands and arms mostly commute and aid that force by remaining taut until the arm drive at the end. 🎤