All you do is put a small light, like a LED at the focus of the parabola. The light coming off the parabola is now parallel, at least in theory. You then aim the cass. at the parabola and either knife or ronchi test at the focus of the cass.
The parabola has to be of very good quality, since any error in it will also add into any error in the telescope under test. The parabola should be as large or larger then the 'scope your testing but if it's smaller you can move the cass. so the edges then the middle are illuminated and get a good feel for thr over all correction.

- Dave