Imagine: a parl vote is held in early feb to authorize the PM to trigger section 50
If parl says cool, everything is fine.
If parl says fuck you, the PM can call an election, and say that only people who promise to vote to authorize are allowed to be Conservative candidates.
That election would happen on the 20th of March
Giving time for the new Parl to meet and vote to authorize and trigger section 50, "by the end of March"
(this will probably be a much longer post on the real blog later)