If you've flown anything like the flyboard - it's quite a bit like snowboarding where the balance required from your feet quickly wears out your leg muscles.

Controlling power from your backpack doesn't wear the body down nearly as much.

[1] Flyboard - https://www.youtube.com/watch?v=m4Bm3cs9TFo

I think he just moved the leg thrusters up to his arms, making two on each arm. At least that's the way it looks from this pic [0], where he has only one thruster on each limb. He may have had the backpack all along, but I'm not sure.

[0]http://i.imgur.com/YTz3Kza.png