to me it looks like it's tied off underwater on the port transom tiedown loop - that's alot of water blast blowing straight out and a funky rebound happening with the machine....

Ahh Reverse , thats would make more sense... I never had one with silly reverse.. I still don`t see how he defies physics and rebounds towards the dock though

There used to be a guy on the river that had tapped the nozzle on a 500hp jet boat for a fire hose. That thing could do some serious damage, but was mostly used to create a shower in the skinny hole on a hot August day. As I recall it took two men to hold on to the hose at about 1500-2000 rpm