For me personally, to know that folks out there are trying to kill rats older than young pups by whacking the animal's head against a hard surface makes me cringe. I simply cannot believe that 100% of the time, every time across the board, it causes instant death without any problems.

I have always read the blow to the head is to be done with neonates, where a blow to the head crushes it completely and instantly because it is very soft. I actually think crushing the head on a neonate is quicker than freezing or CO2.

I really hate to read when people try to do it with rats larger than pups, where the bones are harder, the animals are bigger, and the margin of error is increased.

Cervical dislocation is perfectly acceptable by a highly trained supervised person, and only on rats under a certain age/weight. Many guidlines say <200 grams, but 200 grams is almost an adult rat in many cases, and jerking on their body like that can easily lead to broken legs and tails rather than proper cervical dislocation.