Top explanation there Sparrowhawk! - I don't think either John or myself could have said it any better!
As you say, to raise the compression ratio back to where we would need it for reasonably decent performance, the piston in effect needs to move a little closer to the top of the cylinder/combustion chamber... we are hoping that by essentially doming the top/centre of the piston, it will create the necessary compression, while still allowing clearance for the valves, and at the same time not buggering up the whole efficiency of the chamber itself... (suck squeeze bang blow).
The only other thing I would add to give Greg (or anyone else) a clearer picture of the mechanics of the conversion is that in using a longer rod with a shorter stroke but in the original cylinder/barrel - we are effectively no longer using the lower part of the cylinder wall that was swept with the stock crank and rod...
Hopefully that clears up any confusion!