I've actually never heard the term. I'm sure it is some way of describing the use of thumb and finger with the "banjo" technique. Perhaps to include a little more of the letter may provide some insight. Sorry I'm not much help.
In Clawhammer terminology, there is a technique called double thumbing, which refers to sounding the thumb string after every melody note. It is often used to add variety when there are several melody notes in succession on the first string. In Minstrel terminology we would call this a double strike, but without the drop thumb.
Bob Flesher posted his answer to the original question over on the BanjoHangout, for those of you who might be interested: http://www.banjohangout.org/topic/269700/#3402885