When a player passes he gives the opponent one stone as a captive.
The game ends with two consecutive passes.
Each player's score is the number of captives he has.
Other rules are as in regular go.
Bill: It appears that he came up with Zaru Go before Dec. 24, 2000, which is the date for the counter on his web page.
RobertJasiek: Zaro Go seems to allow pass fights, so we created different things. His is a Go variant - not Go. The idea of scoring / counting only pass stones was developed independently by either of us, it seems - but for apparently different purposes.
See also Zaru Go Problem 1.