That's right, they work on 'knee flexation' whereas SLDL, RDL, GM, DL, etc are for 'hip flexation' the only alternative is 'leg curls' which is hardly a replacement.
I use a sit up board, hook my ankles under the pad, and my feet go through the gap, because conveniently this particular situp board has one.
Watch the following clip to see the 'home' version:
http://www.youtube.com/watch?v=57pv_1j4dH0