Newbie with question about transmission brake

[purzell]

Hi, I'm new to the forum but have been reading and researching here for a while. I'm currently shopping for a truck and went to see one 1966 GMC that has a transmission brake. I can't find much if any info about this type of brake except for in this person's thread:

http://67-72chevytrucks.com/vboard/s....php?p=4958220

I know Google is my friend but there must be a name for this brake other than transmission brake because I'm generally good at finding info online but not in this case.

Any leads are appreciated--want to know how this brake works and what it does. Thanks!