Here is a thread on them:
https://www.strongfirst.com/forums/topic/strand-pulling-missing-strength/
People who use a chest expander properly are noted for exceptional strength and development of the shoulders, upper back, and arms (especially triceps). Since the handles are held in either hand, or in the case of curls, one hand, while the other handle is being held down with the foot, the chest expander is not designed to effectively train other movements, although with high tension, the entire body is "worked" to some degree.
Many guides give a variety of recommendations, and many have a bodybuilding focus.
Scroll down to the PDF downloads:
http://yoga-horizons.com/2011/04/strand-pulling-guides-by-fatman-and-brad-reid/
Look in the table of contents, but the entire book would be good to read:
http://www.sandowplus.co.uk/Competition/Inch/ManualPhysicalTraining/mpt-intro.htm
Modern site:
http://www.expanderkrank.de/en
http://www.sandowplus.co.uk/Competition/Danks/ChestExerciser/chestex-intro.htm
http://www.sandowplus.co.uk/Competition/Bonomo/powerplus/pp-intro.htm
Keep in mind many works are for a general bodybuilding/strength goal and for those who may not have any experience. It is better, I think, for most who have some experience and goals already, to first try out the various pulls to see what one wants to do, and select the ones which are good for your goal. Some, like the Back Press, may be very interesting, as one can press an exceptional amount of resistance and the effect on the triceps of the arms can be dramatic, and others have stated it is great for pushup strength. I do not do the Back Press, but I believe their testimony.