The act of reaching back to provide vigorous hand relief to a good mate who has just given or is in the act of giving you a shoulder massage. This is not an act of homosexuality, more a fine way to say thanks to a friend doing you a great favour.
The technical term for this piece of total mateship is The Reciprocating Massage.
Mike the Fireman reached back on his friend Pete the Volunteer Fireman who was working out the knots in his tight powerful shoulders. The reach back left both men feeling a reciprocating sense of relief.
When someone who is being penetrated from behind reaches back to manually simulate their partner's buttocks or anus. Often performed out of extreme gratitude, it is possible but challenging to combine with a reacharound.
Roger got so randy being rogered by Randy that he gave him a reachback before he was offered a reacharound.