W. B. Toomer & Bruce R. Welch described the Spelean Shunt as “a new method of rigging a trailing ascender as an abseil safety device” in Nylon Highway #9 (May, 1978). That article cites Neil Montgomery's “Single Rope Techniques,” im which Neil credits them with publishing an article about it in 1977 in the article “The Spelean Shunt technique” in the ASF Newsletter.

The Spelean Shunt is improvised in the field by attaching a carabiner through the eye of a non-spring-loaded Gibbs Ascender. Webbing connects the eye to the seat harness. The shunt rides above the rappel device, and ideally the weight of the carabiner keeps the cam open. When the caver gets out of control, the extra drag on the ascender supposedly overcomes the weight of the carabiner, and the cam locks.

I don't think anyone has ever shown that the shunt will always lock off when it is needed. One way to test this would be to use the three-rope rig described in my internet post, I doubt that the tests will be successful. One thing I do know is that the @#$% thing locks up often enough when I don't want it to. Although easily released, it is such a hassle that I never use the Spelean Shunt.