So thankful that Alexa told me about the Sterling Hollow Block (shown below) during my private ice lead lesson in 2016. After my lesson at Cathedral Ledge I went straight over to IME and bought one. It's just $12 but this "$12 piece of gear" saved my ass when rappelling off the Eaglet Spire in Franconia Notch, NH!
When going over the edge both my feet slipped out from under me and I let go of my brake. It was instant and I didn't even realize that I had let go until I was stopped by the face of the cliff. I looked down and there was my Sterling Hollow Block - it had caught me like it was supposed to! I sent Sterling an email explaining how their Hollow Block saved my life and they in turn sent me a bunch of cool swag.
I personally set this up with the Sterling Hollow Block tied in an autoblock to my belay loop via locking biner, and with my rappel extended on a sling. The Sterling Hollow Block is always on my harness. Always back up your rappels! Sterling Rope of Biddeford, Maine, USA.