When your working in Photoshop, you're often zoomed in very close and need to move the viewing area around. Some go to the toolbar and select the "Hand" tool, those extra savvy folks hit the "H" key (h for hand) to switch to that tool. The Jedi Photoshop use holds down the spacebar. This turns turns whatever tool you're using temporarily into the hand tool. As soon as you let up, the tool reverts back to what it was. This makes working on detailed areas with the brush tool so much faster and smoother.