Let me tell you how the normal pin works, and how this works. The normal "pin" is just a bolt on one side with a lock washer and washer on it. You insert the bike rack in your hitch, and then tighten the bolt into the Passenger side nut of the rack. So your hitch is attached on one side with this bolt going through the pinhole on your hitch and screwed into the nut of the rack inserted in the hitch.
The stl2 is that same bolt, but longer with a mushroom head (let your imaginations fly). It still screws into the hitch, but goes out the other side of the hitch, and a lock goes onto the end of the bolt, locking against the aforementioned mushroom head.
There is a LOT of bolt showing even after you put the lock on this thing. So if somebody wants to cut the bolt and steal your rack, they will. But we all know locks are for the "honest" people anyways.
This thing costs 45-50 bucks! Ridiculous. Thule should be ashamed of themselves. It should cost at most 30. And they'd still be making 25 bucks off the thing.
I do like the fact that even if the bolt unscrews itself, the rest of the pin goes through my entire hitch. Nice piece of mind when I drive 3+ hours with my bike on the back of my rig.
Cheers!

The lock helps prevent problems with the bolt coming lose and also gives peace of mind against theft of the carrier.

Thule Snug Tite Lock (STL2 lock with lock and key)
Have a THULE rack that is from 2003 and it works perfectly.

Nothing fancy.....bolt it on, prevents horizontal sway and locks to keep the hitch from easily being removed. The pin though is longer than needed and sticks out the other end a little too much. A second spare key would have been nice. A little over priced for what it is but you are paying for the name and the quality.

Brought this brand new from BackCountry.com, and it's very easy to install.
I did feel safer using the longer screw came with the lock, than the original that came with the bike rack about 1/3 of the length compare to the lock screw.
The lock is very simple and very secure, all you have to do is tighten the screw thru the hitch with the rack, and put the lock on the other side and turn the key to lock it, that is all to it.
Even though the rack base is secure with this lock, but the rest of Thule rack can still be stolen. I don't know why Thule want to sell this lock to lock down the racks, but use the regular nut for the rest of the racks, that can easily be removed with the proper socket.
What I would like to see is Thule use those secure nut similar method that use on car wheel, that require a right key to remove.

Hope this help.