Because I don't want him to have pawns on d4 and e4 controlling the centre.

Seems to be wanting a Queen's gambit variant or a Benoni possibly?

Of course, if played immediately it does lose a pawn, and I am going to prepare for it with Na6-c7, Bc8-d7 and a6 so it doesn't lose a pawn when I advance it.

Plans can be long or short term. The plan Charles outlines is short term, just 4 moves deep, with the view to undermining the pawn at c4 and loosening White's hold on the centre. There is very little White can do to cut across this plan, but there are probably better things for White to do with his next four moves anyhow.

I would not be so sure about the very little white can do. I am sure that white sees the situation but is choosing the best moment to respond.

Look at all that sweet sweet space that white has! Of course black's bishop on g7 looks great and there might be an argument that white is slightly overextended, leading to a nice even position with good dynamic play. I am normally on the black side of these positions and if I can play e6 at the right time I feel pretty good and otherwise I often find myself with limited options.

Focusing on the Q-side for the moment, Charles is demonstrating the correct method of forcing a pawn forward (...b6; ...a6; ...b5) in the circumstances that have arisen. It is a modification of his plan occasioned by White's throwing forward his a-pawn.

Never go into battle having only one plan of attack a wise man once told me.