The thumb disks tend to be on knives that aren't very "flickable" with the thumb. Emersons for example can't be flicked open like many Benchmades (studs)are designed to be. The disk allows a similar opening to the Spyder hole in that the thumb guides the blade all the way to the opened position. Don't get me wrong, Spydercos can be flicked open, just not like Benchmades. I personally like Spyder holes. Also, not all thumb studs are on thumb flickable knives.