I'm curious how a thumb release can be used as a BT.
Any release can be shot using the back tension "technique".
What that means is that any release can be made to fire
by squeezing the middle back muscles.
Michael Braden teaches in his DVD, "Straight Talk with Michael Braden"
how he shoots his wrist strap Carter Two Shot release,
by just squeezing his middle back muscles.
No squeezing of the trigger finger. He just makes his index finger into a hook and pulls his entire release side hand, wrist, arm and elbow
using his middle back muscles. Just pull with your back muscles to make your release side elbow move behind your head.
This same technique,
make your release side hand into a "claw"
and pull with your back muscles,
and you can also make a thumb trigger release fire.
If you read Larry Wise's book,
then you will see lots of pictures and lots of explanation
about how after you hit full draw, and hit your anchor,
you relax the release arm muscles,
relax the release side shoulder muscle,
and squeeze your middle back muscle
to move your release side shoulder blade towards your back bone,
and try to move your release side elbow tip
back behind your head.
No squeezing of the release hand required.
It does work.
I was just helping a local AT member today
at my home range with "back tension" technique
using a thumb trigger release,
as well as showing him the finer details of
the "power triangle".