So those water tanks are representing user’s body/ki capacity, in forms like: ssj,2,3 your ki always drains, even when you’re just standing still, the stronger form, the more it drains, but God Ki works differently, because you CAN set your multiplier of this form by also setting ki drain, those taps represent you being able to use more or less power of this form so you don’t need to use full power of this form (unlike other forms). Ok I don’t know what to say more… Ummm… ok I’ll just leave it like it is 😛
Actually you don’t need to use full powah even in SSJ. remember when Goku/Gohan power up against Cell. They power UP, which means they weren’t at full power though being SSJ. Which means they’re aren’t doomed to go full power or stay in base, they still can control their output. Oh I’m too lazy to watch the video so I probably say shit.