The move applier is a long, but relatively straighforward piece of code.
It takes a move, records it in the undo buffer, and applies it to the current board state.
Each of the possible types of move are covered with a piece of code, i.e.

Pawn moves

Reset the draw50 counter

Check for en-passant, removing the enemy piece

Check Pawn promotion

Move the pawn

If the pawn move is two squares, set up the en passant square for the next go

Castling Moves

Check all squares involved in the castle are not attacked, as one can't castle through check

Move both the king and the rook involved

Update the appropriate king position data

King moves

Clear all castling flags

Rook moves

Clear the approprate castling flag

All moves

The piece is moved in the board arrays to make the move

Left in check?

We then check to see if the current player is in check, and if so we mark the move as invalid

Three move draw

The three move draw test is based on that found in TCSP as described by John Stanback. It's a little difficult to describe, but I'll try and put up some more details here later.

If the move was invalid, we call the undo move function to reset the board state to how it was before the move.
Finally, we return whether the move was invalid.

The Move Undo-er

This is pretty much the same as the move applier, but in reverse.

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