Thick VM with thin SAN?

Hello,

We are currently running a thin provisioned SAN back-end with a thick provisioned VM environment.  I'm trying to understand how this works / the logic behind it.  Is the SAN essentially saying "I'm going to give VM a logical block, but I will only account for what is used" and then the VM env is saying "oh I have this whole block of storage, I'm going to take it all"?

That is the scenario that I laid out in my mind, but I'm looking for confirmation as to what is actually happening here.

Thanks in advance!