The parties agree to the establishment of five year fixed-term parliaments. A Conservative-Liberal Democrat coalition government will put a binding motion before the House of Commons in the first days following this agreement stating that the next general election will be held on the first Thursday of May 2015. Following this motion, legislation will be brought forward to make provision for fixed term parliaments of five years. This legislation will also provide for dissolution if 55% or more of the House votes in favour.A 'Parliament' as the term is used here, exists between general elections. It can, but need not, be dissolved on a vote of no confidence in the government. Parliament cannot bind its successors, but it can bind itself for its own duration, even under Dicey's rule. Assume also that the 55% rule applies to the current Parliament (which it may not!).
In the current Parliament, this means that David Cameron cannot go to her majesty and request a dissolution until May, 2015. That ties his hands. It also withdraws the royal prerogative to grant such a request for at least five years. However, if there is a vote of no confidence, that does not mean that there will be a general election. The Liberal Democrats will preserve their position as the pivot in the House of Commons for the full five years until May 2015. Assume that the 55% requirement will apply in the current parliament as well as in future ones. The Conservatives lack the ability by themselves to dissolve Parliament (55% = approximately 358 votes, depending on how one counts). Thus, if the Liberal Democrats leave the coalition and the Conservatives cannot pass supply or other important legislation, her majesty will call on the next largest party to form a government. That will be Labour. This strengthens the Lib Dems' hand when they are negotiating with the Conservatives. It, too, preserves their position as a pivot in the House of Commons. This takes power from (a) the queen; (b) the executive; (c) a party in the House of Commons that has less than 55%. It gives power to everyone else. If you're trying to force people to get along, that's a good thing. If you want an elected dictatorship, it's a bad thing.
No matter what, it's another stake in Dicey's heart if the legislation is passed, presumably with a supermajority requirement on its repeal, which limits every future government's ability to dissolve but also narrows the royal prerogative. At the same time, it's all in the details. Will there be a super-quorum requirement? How will the 55% be counted? Will the speaker and deputy speaker and Sinn Fein be part of the requirement?
This historical background just in: Constitution Unit Briefing Paper on 55% dissolution requirement.