I had some bad luck with a come along once. I put some pretty good tension on it, and made my back cut. Maybe no notch was my downfall. Figured I could get away with it, since the tension on the line. As soon as it started to fall, the tension disappeared and the tree went where it wanted to after that. It didn't hit anything fortunately, but it taught me a lot about how not to tension a tree felling line.
Yeah, you only want enough tension to hold it until you finish the cut. Too much and you can break the hinge or pull the tree right off the stump if the line is not high enough in the tree.
For that matter, if you use a bore cut instead just making a progressive back cut from the outside in, there is far less chance of losing control of the tree. Dead trees are the worst. If they are dead and dry, not enough hinge wood can make it snap right off as soon as you start to pull and the tree will twist and fall where ever it has a mind to.
A few years ago, thinking about that moment, I had my neighbor help me drop a tree out back. He used my tahoe, and I asked him to keep lots of tension on the rope. That way, when the tree would start to fall, he could keep tension on it all the way to the ground. That's when I found out what happens with too much tension. About halfway through the backside cut, the tree split from the cut up to about ten feet up, kicking out severely. I remember that split portion flying past my face at about mach two, feeling the cold wind from it about two inches from my chin. That was a pretty good shakeup, but no physical harm done.
Barber chairs can kill you. Again, too much tension before the back cut is done can pull the tree apart. Using a bore cut usually can prevent it from happening.
Couple years after that, same uber cool neighbor had some trees professionally dropped in a pile like pickup sticks, and he'd cut them up later. Yeah, he's smarter than I am. Anyways, I noticed a day or two after that he was getting around kind of slow and awkwardly. I asked him what was up. He said when he was cutting, he remembers getting just about through a cut, then waking up in my yard about 15 feet away with the saw idling on the ground next to him. He has no clue how long he was out. Apparently, the trees must have been stressing each other the way they were tangled together, and one of his cuts relieved the tension in a really bad way, and tossed him like a rag doll. Darn near busted him up pretty bad. Never mind the rotating chain on the saw flying through the air he no longer had control of, unconscious and all.
Trees that are on the ground either from being cut or being blown down can be far more dangerous than trees that are still standing, especially when there are two or three on top of each other. There is a lot of tension in some of those limbs. Cut the wrong branch and not only can it whack you pretty good, it can cause the whole tree to roll over taking you with it. I've seen this stuff happen many times.
The other thing to realize when cutting trees that have fallen is that chaps are not going to protect you all the time. Chaps are for cutting with the saw held below the waist, as in felling or bucking firewood. When trees are on the ground, you frequently have to cut with the saw held up high to prevent something from springing back. I always wore a protective shirt when doing storm damage. Just like chaps, with the protective fibers in the sleeves, upper chest and shoulders. Its not something to fool around with for sure.