The 6x17 backs for 4x5" are extension backs, like a 5x7" back for a 4x5" camera (Wista makes one, for instance). They have separate groundglass viewers with the same amount of extension as the film back, so you attach to the groundglass viewer to the Graflok back, compose and focus, then switch to the film back and shoot.

They generally work with lenses in the range of 75-150mm, due to vignetting issues at the long end and minimum focus distance and lens coverage issues at the short end, but that's not too different really from the dedicated 617 cameras, and it's possible to use a longer lens, if you don't mind shooting a format like 6x16 or 6x15.5.

I have the DaYi 6x17 back. It also has masks for use as a 6x12 or 6x9 back, which is pretty standard on these Chinese backs. If you do a search on "dayi and 617" you should turn up one or two threads on these backs, and I've posted a few test shots.