You guys are missing the obvious.
When he's straining, he's trying to focus, and also he's having to lift a submarine out of water.
The extra pressure of the water means it's much more effort to move the sub. Think of lifting something out of water vs lifting it up in the air.
Then when the subs is out of the water, there is lots of water still all over it, which all falls off, reducing the weight of the sub, meaning through both these things, plus him being more relaxed having achieved the difficult part of the task, means that he's much more relaxed and controlled in his holding of the sub, and able to swing round and move it to the other side of the jet as well.
Quite a simple explanation really.