The removal does trigger the command
The "breakaway" part is a USB cable. The actual USB device is at the other end, right by your belt etc.
The point is that USB sockets are sometimes a bit stiff and USB sticks fragile, so you want to make certain that your security key is definitely going to disconnect and trigger the udev rules.
Otherwise the guts of the $5 USB stick might just stay in the socket while the chain and case dangles from your belt, until the miscreant finds their rubber ducky.