This knife, upon a successful kill, will make the enemy's body disappear, along with the victim staying silent when killed. This also puts you in disguise as the victim. If you kill them via Backstab, there will be no kill report on the screen. However, the weapon still drops meaning if you have low cloak and someone knows you are a spy, quickly grab the weapon and cloak.
The downside to using this knife is, due to the method of disguising, your disguise kit is disabled.