I have several of the sheaths and slips that are made with the clip. I have a clip sheat for a small bird and trout fixed blade and 2 clip slips, as they are called, for two of my trapper knives. The clip slip is basically taking the standard knife slip which is fairly common for slipjoints as a way to keep them from getting dinged from coins or keys in your pocket but also help keep the knife facing vertical. The clip is sewn on so that the knife can be clipped to the pocket, or where else you would like it, just like a modern folder with a clip.
I've had the clip slip for close to a year now I think with a boker trapper and frequent carry around the house and have really liked it. The slips come flat so they have a fairly snug fit but after time they will take the form of the knife that is frequently in them, as leather tends to do. This can lead to the slip being a little loose over time but it's definitely not bad. It does mean that the slip may not work well with multiple different knives.
I do find the clip slip works better with a boxier bolster as the rounded bolsters on my #48 gec can be more difficult to withdraw from the slip while it's clipped in my pocket. If you do tip down with an opinel I think the clip slip would work really well.