If you need to test one line in the middle of a bunch of lines, it can be hard to get your Butt-set clips on the 66 block.
The 66 Block Test Shoe lets you test a line by removing a 66 Regulator, 66 Lightning Protector, or 66 Line In-Use Indicator, and placing the Test Shoe in its place. Attach your Butt-set to the clips on the front, and push the red switch to open the line and disconnect the equipment during testing.
Remember that the phone line always comes in on the left on an RJ-21X block, and the CPE equipment is always on the right.