2010 Bikes

Put a tube in a mould of the shape you want it to distort it into then force a hydraulic fluid inside the tube making it expand into the mould.

Yes, that's the theory.
And the way Giant does it (sometimes).
But after this "technology" became a marketing name, they used it for all kinds of deformation.
Hit a tube with a hammer, clamp it in a vice, slam a solid piece of metal into the end of a tube and call it hydroforming.