During a division, MPs must pass through the division lobbies to register their vote. If an MP is not able to get to the division lobbies because they are ill they may still vote under a procedure called "nodding through".
If the party whips decide that an MP qualifies, his or her vote can be counted as long as he or she is within the precincts of the Palace of Westminster.
Their name is passed to the clerks and tellers so that they can be nodded through without walking through one of the voting lobbies and their name is included on the voting register.
MPs who are very ill are only required to go to this trouble when the issue being voted on is of vital importance and the government's survival is at stake.