Reverse button go is just like button go, but instead of the first player to pass, the last player to pass takes the button (worth 1/2 a point). Two consecutive passes end a game.
Under AGA rules, pass stones are used, in combination with the rule that white must pass last, to achieve a score by territory counting that is the same as the score would be by area counting.
The rule that white must pass last is somewhat unpopular it seems, introducing an asymmetrical game end condition (the game ends after either two or three passes, depending on who passes first).
If, however, you end the game after just two passes regardless of color, you get a phenomenon called "pass fights", which go something like:
If you compare this with a normal game end:
Then white has gained a pass stone, and thus a point, by the ko-threat and response exchange. Of course, instead of passing (step 5) in the first sequence, black can also play a ko threat, and get:
Which would rebalance the number of pass stones, and hence the score. But again, white need not pass on step 8 here...
The above is what is called a pass fight, and one extra point will hang in the balance depedning on who has the most ko threats.
With reverse button go, pass fights are useless, because the outcome will be identical:
Black gets 1 point, White gets 1.5
Black gets 1.5 points, White gets 2.
Black gets 2 points, White gets 2.5
So in all cases, white gets 0.5 points more than Black, and there is no incentive to start a pass fight.
With reverse button go, you can use territory counting with pass stones to get a territory scoring result, but with the advantages of area scoring. Due to the pass stones, moves inside your own or your opponents territory after all dame are filled do not cost points, so many difficult positions (like bent four) can be resolved by play.
Bill: The half point button, in conjunction with the pass stone, means that it does not matter who passes last. So two (or three, if you are inclined that way) consecutive passes can end the game. :)
Since it does not matter who passes last, we can, as the current rules do, suppose that White passes last. In that case the territory score on the board will be the same as the area score, and White will get a half point. So if the komi is 7, the result will be the same as the area score with a 7.5 point komi.
There is no reason not to play Two Button Go, combining the effects of the two buttons. :) Usually the buttons will cancel out, as the game will end with consecutive passes. We can simplify two button go with this rule: If the same player makes both the first and last passes, she gets one point. Or, since we are using pass stones: If the first player to pass also makes the last pass, she does not give a pass stone for the last pass. :)
Herman: Heh, I totally missed that it does not matter who passes last. So indeed, the score will be the same as area scoring, not territory scoring as I claimed.
I do like the idea of Two Button Go though, especially when simplified as a rule about first/last pass.
Two Button Go, it seems to me, combines the advantages of territory and area scoring. It is granular and easy to count, like territory scoring, yet allows difficult situations to be played out on the board without special extra rules. :-)
Bill: Yes, a nice synergy. :) Many thanks, Herman!