I never heard of a snare that tighten of it's own accord but I've used snares with locks that prevented the snare from loosening when slack occurred. I also used a stop on land sets which prevented the snare from tightening beyond a certain diameter. This allowed deer and moose to shake off the snare, hopefully at the site, rather than walk off with it and die of complications of being caught in the snare. More importantly at the time, it prevented the loss of the snare which was an expense to be avoided.
The stop prevented the snare from choking the trapped animal which allowed for its release if a non-targeted animal or one with a less than desirable condition pelt. Surprisingly, the non-killing snare usually caused less pelt damage as an animal that can breath did not fight against the snare as violently as one that was being strangled.
I caught a few dogs too, most were used to being tied out and stopped long before the stop when they realized they were caught. I can recall two that were down to the stop but suspect they were feral as I never saw them before and they did not have collars. They were also more viscous than anything else I caught, even wolves.
As such, I am much more concerned with Conibear traps than snares. Conibear type traps are designed to kill quickly and they do a very good job of it. I've used Conibears but always with trepidation as they are not selective.
