gravity knife

Short definition from Wiktionary

Noun

1. 1
   A type of knife that with the press of a button, allows the blade to drop out of the handle, and then quickly opened up full, for quick use, also allowing it to be flicked open with a flick of the wrist: A folding knife of this type.
2. 2
   A type of knife that with the press of a button, allows the blade to drop out of the handle, and then quickly opened up full, for quick use, also allowing it to be flicked open with a flick of the wrist: A telescoping knife of this type.