welcome aboard, John
here's my best attempt at an explanation!
'push pulls' pull the changer fingers to raise and push them to lower
'all pulls' pull to do both. they do this by having a joint in the changer fingers (the bits that move back and forth). so when the top half of the finger is pulled it raises, and the bottom half lowers. because the finger pivots on the joint and pushes the top half in the other direction
hope this makes sense, it would if i could show you a pic.
anyway, the obvious difference between a 'push pull' and an 'all pull' is that there's a joint in all the fingers of 'all pulls' but 'push pulls' have solid fingers.
hopefully someone who knows what they're talking about will post soon and clear this up! Richard, Bob...?