Undoing Photoshop

I’ve used Photoshop to some degree or other for many years now, since version 5 and the one thing that always bugged me, was hitting Ctrl+Z only undoing the last operation and would toggle that undo as an undo / redo function.

I was working on a sketch earlier and for some reason, whilst Ctrl+Zing, I hit also Alt; what happened? It started to undo not just the last action, but the one before last too, and the one before that and so on!.. so whilst this might be common knowledge for professional Photoshoppers, for myself (and possibly others), this was a most pleasant surprise and makes working with a graphics tablet especially, a lot simpler!