"If-need-be" allows you to indicate that you can make a selected date 'if-need-be' (i.e. its not the best date but you could make it if you have to). If-need-be must be enabled by whomever created the poll.
To vote 'if-need-be" on a poll that has this setting enabled, click twice on the voting option. The first click votes yes, the second click votes 'if-need-be'