If I understand this right (and I doubt I do) they are voting on the deal minus the controversial backstop yet if they vote for it they we will leave on the 22nd May. They're basically looking to pass a deal that doesn't exist.
Not quite right - they will be voting on the withdrawal agreement which includes the backstop, but not the political declaration.
That is, voting on the legally binding arrangements for the next two years of 'transition' (and the fallback position if nothing further is agreed during the transition) but not voting on the non-binding 'wish list' of items to agree during the transition period in order to sort out the long term relationship.
Essentially, voting to have two years of almost everything staying the same while we sort "stuff" out, with the caveat that we stay more-or-less in that position indefinitely if we can't sort "stuff" out - but not voting on what that "stuff" actually is.
BBI45, Rkolsen and Brekkie gave kudos