Not to plug a particular brand name, I will use what ever works, but Emerson Knives have a technology called the “wave.” I carry an Emerson Karambit on my weak side as a back up. It is a stout folder that has a large finger loop to draw the knife. When drawn, the “wave” catches on your pocket and automatically opens the folder.
It is a slick system and Emerson offers several different varieties of folder and fixed blade knives.
Relying on a knife for back up is just like counting on your primary weapon. It takes training and practice to access them both quickly and cleanly. One should practice drawing the backup weapon on a regular basis, whether it be a knife or gun, to keep the muscle memory alive and sharp.