Nope its not elasticated, its quite rigid although reasonably flexible.
You'll really struggle to put it on too tightly but it is quite a nerve wracking thing to do for the first time
i've never had a bird show any sign of breathing distress when fitting them, even though it feels quite tight sometimes they will sit down for a while feeling sorry for themselves till they get used to it but usually they start jumping about like a mad thing.
Put one hand around its neck and lift all the cape feathers up then attach the collar as close to the base of the neck as you can, its probably easier with two people.
Make it as snug as you can, obviously without choking the bird
i can get my pinky under mine but can't get it past the first joint and it shouldn't be able to move easily up and down the neck.
Make sure the overlap on the outside is at the back or they can take it off!
When you fit it put the bird in a pen where it can't hurt itself on anything.
they do try and back out of it for a while until they get used to it and can flap and jump a lot which is quite alarming the first time you see it!
I've put them on a bird before and 5 minutes later its as if nothing has happened but I've also had some still trying to do the backing out thing the day after it was fitted.
If it crows and its still quite loud, tighten it slightly more until you get it to the right place.
Here's a video of someone who has just fitted a collar
https://www.youtube.com/watch?v=r2Sq77UBdJc