Easy, now--see the man with the wood post? He's crushing their heads. It's brutal, but pretty effective--brain death. The snakes are still moving, but they're brain-dead. Reptiles can 'live' for a long time with no head--they certainly aren't feeling pain in such circumstance.

It's not impossible that he might miss one, or not do a good job, but it's not very likely that the snakes are feeling pain after their heads have been smashed. Most of them aren't feeling anything, even if their bodies still move. The guy doing the inflating would send them back to the killer if they were struggling to get away--he's got no desire to be bitten or fight with one. They are therefore entirely dead, or unconscious.

Movement doesn't signify life in a snake. This isn't a pretty scene, but they aren't skinning the animals alive.