Tony, that's a good question. I never got around to documenting that as it was a late addition. It does indeed vary the amplitude of the TMCC signal. This allows you to "tune" the signal to minimize any interaction with DCS and also to reduce the amplitude if you have issues with stuff like LCS Sensor Tracks.
Even with the signal turned down to minimum, the buffer is still providing a substantial benefit, it gives the TMCC signal much lower drive impedance so that it can drive more track capacitance. There are people that run it at the minimum with good results, it fixes their signal issue at that level.
The way to use it is simply turn it down as much as possible while still solving any TMCC signal issues, that will insure the maximum compatibility with device that may react poorly to too much TMCC signal.