What does mast rake do?

Actually the objective is completely different. Aircraft flying near the speed of sound use swept wings in order get the wing and engines working behind the shock wave generated by the nose cone. See http://en.wikipedia.org/wiki/Swept_wing for a longer explanation. This is not relevant to sailboat design just yet. 

The only reason they would be letting the kicker off is to stop the boom burying as they see-saw around in the gusts or to get under the boom and stop sticking in irons in a tack. The Contender get pretty uncomfortable to sail with little kicker in a breeze as the sheet loads increase significantly and when you sheet out in a gust you dump all the power at once (not good).

Maintain some curvature in the sails in the light stuff, don't have bags in the sails but at the same time, don't have them flat. There are two schools of thought with light wind settings, but flat sails to reduce drag theory also kills all lift too and is very slow. Maintaining some curvature maintains a profile optimum for generating lift. High Performance Sailing by Bethwaite and Sail Performance by C A Marchaj, both cover the theory in great depth, Sail Performance is more specific. Best to read a book about it instead of trying to understand bits and pieces from a forum if you are really interested. We don't adjust mast rake on the 49er as we have a fixed forestay but in the light stuff we don't bend the mast extensively to move the centre of effort aft as is the effect with raking as it is a depowering method, probably why Alex was saying keep the mast straight initially. If you bear in mind what Ian was saying about wings on the 747 and Concorde, it's a good analogy of sail shape. The 747 wing shape with more curvature is designed that way to generate maximum lift to carry a high payload whereas concorde's wings are very flat with the aim to minimalise drag. The 747 shape will help on light days to maximise lift at lower speed and on the windier days when you have plenty of lift the aim is to flatten sails off to reduce drag.

Garry,

A basic understanding of vectors demonstrate that the faster you go the higher you point.  so why should easing the sheet a little stop me pointing, except in the short term whilst I accelerate?

The practice of increasing mast rake in more wind is about giving the wind a different aerofoil from that used at lower wind speeds.

Rake should not be confused with bend. 

Bladed sails, caused typically by lots of cunningham and less kicker when windy are fast around the course.  This may actually be due to making a lightweight boat easier to sail as the top of the sail is controlled better rather than actually increasing straight line speed, but they are fast around the course .