I build in a natural thumb break on my standard kydex sheaths....because I want a good solid lock-up and yet still be able to draw the knife without too much effort.
But on these, with the blade always pointing straight down....they are terribly easy to draw....even 2-finger draw....and re-sheath. They function just like a leather pouch sheath, except that they retain the knife a LOT better - won't fall out if you turn it upside down.
The trick with these is that I don't put rivets up high near the guard...yet keep the "front piece" of kydex tall. Those that do kydex will understand immediately.
What you end up with is a stiff/sturdy sheath (because of the 3 layers) with a "wide mouth" for drawing/sheathing the knife. The rivet acts as a pivot point when drawing the knife. The farther away it is from the "mouth" of the sheath, the easier it is to remove the knife.
And the design of most of my handles allows for a fairly good "grab" - plenty of places for the kydex to "grip" the handle and hold it in place.
I guess what I'm trying to say is....(in waaaay too many words)....this sheath doesn't need a thumb-break...and wouldn't really benefit from one.
1 - it draws/sheaths easily (rivets are lower than standard)
2 - front piece of kydex makes it behave like a sheath with a "cam" in it - holds it in place.
Dan