[Bill]

'two hydrofoils moving in opposite directions '

Not sure if it is important or even true but it feels very different. The rear foil always points at the centre of the front foil. The front foil is free to swivel restricted of course by the bungee which can be adjusted for 'fin stiffness'. If my long term memory can be trusted, it feels like your feet are anchored in place and your body hardly bends. This can't be because when the front foil starts up the rear must go the same direction but the delay forces the undulation with no input from the swimmer. It becomes obvious when you try to wear fins too. The efficiency seems to go way down.