You're right, mac has been using this technique for a while. Actually trying it out right now, it still does this, but you need to move it fast. If you move it fast enough within the triangle it'll stay, while if you move it at the same speed straight down, the menu goes away immediately.

You can also tell it doesn't seem to do it when you move the mouse back to the original menu item.

There is a small delay, so they don't use the Amazon technique.

Oh yes, you're right. You can tell they use the triangle technique though.

It was like this in System 7.0 at least, perhaps 6.x.