In the Helios, the small inflatable tubes with nylon straps are the stretchers or foot braces. They are used to push against with your legs to keep a more upright paddling posture, and they are adjustable in length. An efficient sweep paddle stroke uses a twisting motion of the torso (rotate your trunk pushing the paddle) requiring bracing of the feet. This paddling motion will reduce fatigue of the arms.
When possible, straddle and sit down. If not, use a paddle as cross brace and get your weight centered over the middle of the boat before sitting down.