That video is totally misleading for anybody on this forum, and what he does is unsafe for the controllers we use. It would be better not to show it. Instead, you could describe how to do it properly and safely, or find a video that explains everything, or make your own one and post it.That guys method looks very dubious to me.
The shunt is a resistance device made of wire of a known resistance. The voltage drop across the shunt caused by the current flowing through the shunt is used by the controller to calculate and limit how much current is passed through the controller to the motor. First he joins all the three shunts in that controller together, and then he plasters the whole length of the shunt wires with a massive glob of solder. That shunt is now not a shunt at all. It is now a very low resistance. Other shunt mod videos show a small part of the shunt being coated with solder to lower the shunt resistance just a bit.
I doubt that the mosfets would last long if there were not 15 of them in that device. I reckon the only thing limiting power in that bike is the battery overload protection - or the mosfets blowing.