Right above the foot pads are the handles to pull. When pulled a band connects to 3 paddles in the blue water tank and pulls them in a circle. The seat of course goes slides back and forth. The water tank contains 17L of water, plus a little chlorine to keep the water from gunking up.
One size fits all on this. Resistance is increased by rowing faster. The little computer right above the rowing handles gives strokes per minute, time, distance and all that. I did a bit over 10km in 30 minutes today.