This mod is a very simple and somewhat well known mod to make it easier for you to pull your knife out of your pocket. This type of opener has also been called the "ghetto Emerson opener". If you do not know, Ernest Emerson is the owner of the company Emerson knives. He is credited for the creation of the Emerson opener or wave feature on knives. It's function is so that when you pull your knife out from your pockets that the "wave" catches on to your pants and opens the knife. Fairly simple but it has a restriction as it only works with knives that have an opening hole. This includes all spyderco's and various knives from other brands.
The pictures with the zip tie mod are of my spyderco paramilitary 2 and the knife with the actual wave feature is the spyderco endura 4 wave.