Continuing with the a4 game from the previous page (if I did my counting correctly) ...
My opponent castles, attempting to acquire safety for HIMSELF as well.
But doing so leaves his knight and took in position to be diagonally "x-rayed" by my bishop, threatening free loss of said knight and capture of said rook right after.
He moves his rook to go right behind his knight, which would theoretically protect both ...
... except I want this trade. That knight is a troublesome defender that I want gone ...
... so I can take his bishop out. Yet though I truly, genuinely hate bishops, even that's not the real point. The real point is to force it so he has to avenge that bishop with the only pawn that can do the job, and thus effectively destroy his castle. I now have a straight line of attack on his king.