You can root from the front leg and generate significant power from the waist with the movement that throws your empty leg backwards; it's just rather unintuitive (or was for me anyway).

The wu xing ("five elements") which are the basic forms in xingyi are also usually understood as general formats for training force mechanics, rather than simply the strikes they look like; so that kind of a leg movement can be used, for instance as a takedown like o soto gari.

There's walk-throughs of the xingyi forms here following the 'Crushing' link that might have what you're describing.