Sorry to add more confusion but...
I had avoided the phrase "A tipped stack" because it didn't quite fit at first... but with some kerning, it does. But I still changed the end part again. How about this?
TIP: Knock over any upright piece or stack. A tipped stack separates, all pieces pointing one way.
BTW, if you're short on space, don't be afraid to hyphenate an occasional long word to help make room! It's a perfectly valid typographic device.
It's not that I'm scared of hyphens, I just hate them. They may be perfectly valid, but they're also ugly.