ACLs on Cisco Trunk Link

I am trying to figure out how ACLs applied on a Trunk port work.

We have a Core switch that is doing Inter VLAN routing, and behind that switch are numerous other regular switches. The core switch holds the routing table. This is a very basic configuration.

CoreSwitch
|
|(trunk)
|
Switch1
|………..\
|…………\(trunk)
|………….\
Switch2…Switch3

We applied an ACL on Switch1 going to Switch2 to allow only certain VLANs to pass through.

We tested this concept and apparently it works. My question is, how is that possible as Switch1 is only a regular Cisco switch with no routing enabled on it? The ports are being trunked.

(periods are used for space)