Piece of piss. In a viscous liquid, like water, passing an object through it, if it has the correct shape, and speed, will create a bow wave effect, with a vacuum effect behind it. If the pressure drops enough, surface tension will pull the lower pressure volume of the liquid apart, creating a very low pressure void, which, once the object has passed, will fill, by surrounding water pressure reasserting itself, and collapsing the void. I used to be able to do it as a child, but haven't tried for years. The easiest method I found was...
In the bath, place your hand palm on to the water, with your fingers slightly apart (I vaguely remember something like about 1/2 finger diameter between fingers produced the best results), and push your hand through the water as fast and as hard as you could. IIRC, it worked better as a small child, than as a large one, and, as I've said, haven't tried for years. I may have a go, and see how much water escapes the bath, with my big fat fingered paw at some point, now you've reminded me about it.