I was going to mention that one, from my JROTC days. We got some quality Korean-era CQC training. We wouldn't try to use a full tree, but a stump or post, and use a couple guys to drop the subject in place. It definitely works, as long as the stump/post is of sufficient diameter. I have no idea why he thinks leaning back would be fatal.
Can you escape it by bridging off the ground?
Not if you do it like we did, or if you do it Fairbairn's way on a tree with significant taper. You simply can't move your leg back enough to clear your foot, because there's a tree in the way. Bind the wrists, and they'll find a skeleton wrapped around the post if you don't come back.
"NO. 26. - VARIOUS METHODS OF SECURING A PRISONER
C - The Chair
A chair with an open back is preferable.
1. Force your prisoner to sit on the chair, pass one of his arms through the back
and the other arm around it, and tie his wrists together by means of the
Highwayman's Hitch (Fig. 103).
2. Tie the upper part of his arms to the chair, one on either side (Fig. 104).
3. Tie both feet to the chair - one on either side - with only the toes of his boots
resting on the ground, as in Fig. 105.
4. Gag him, if necessary."
Another securing method, that seems like it'd work well actually. Thoughts?
That's pretty much the box tie plus a chair. Should do fine.
Breakaways from comealongs, #1:
"A - Your Opponent Has Hold of You as in Fig. 108
1. Exaggerate the pain you are receiving by shouting or groaning. Try to be out of step with him, which makes it easier to apply your counter. Only resist sufficiently to prevent him from being suspicious.
2. Do not be in a hurry to apply your counter. The opening will be there every time he puts the weight of his body on his left foot
3. Smartly jab the outside of your right leg against the outside of his left leg, forcing his leg inwards, and break it (Fig. 110), simultaneously pulling your right arm towards you, which, in addition to increasing the force of your leg blow, also permits you to bend your arm and break his hold. If necessary, apply the edge-of-the-hand blow on the back of his neck with your left hand, and kill him"
So here we have a JJJ armbar countered by driving your knee into their closest leg when their weight is on it, while pulling your arm away. I like the little bit about exaggerating the pain you're feeling. I doubt this would reliably break a knee though. In FMA we do some applications of using knee pressure applied to the legs to break posture and if you do them fast there's potential for injury, but to time this perfectly in stride sounds tricky, and seems likely enough that you'd just charlie horse their leg. We also use foot pins with these sorts of attacks, otherwise they can just voluntarily or involuntarily move their foot.
I would prefer to try a sukui nage variation as a counter.