Transonic amp

[roadhouse]

I was reading in the Richard Smith Rickenbacker book about Transonic amps and how to achieve the best distortion...

"Players could achieve the best distortion by running the second channel into the first channel. Although unintended, the Transonic was one of the first amps to make this effect possible without modification."

Does anyone know how this was done and can it be done on any two channel amp?

Thanks, Glen