As far as I understand it the value of CC61 is just the lowest bit (least significant bit) and it is encoded as 0 (==0) and 64 (==1).
And “yes”: it is completely undocumented. Neither the manual of the Peak or Summit documents it in any detail.
In the mono station manual it is also referred as CC(8), but also without any further explanation:
CC(8) denotes a CC pair where the second CC is used to create an 8-bit message for a 0-255 value range.