This rule states what should happen in the event that there is a dispute about whether an overlap existed when the first boat entered the zone of a mark (4 boat lengths) :-

"If there is reasonable doubt that a boat obtained an overlap in time, it shall be presumed that she did not". This dispels the myth that it works the opposite way round. We all understand it is sometimes difficult to see, especially when sailing to a mark directly opposite the bank from a previous mark in line BUT, if there is any doubt about whether an overlap was obtained, err on the side of caution and presume you do not have one.