'Lame' is Betza's technical term for a piece that can be blocked on a
square that it cannot land on.
Say you've got a DD on a6 and another on h6. What do you do when the
enemy king moves to b7?
If they're full Dababba-riders, the one on a6 just moves to g6, and you
still cover the whole rank.
But if they block each other, you're stuck. If he moves anywhere on the
same rank, he blocks the attack from h6 to b6, and the king slips through.
If he moves anywhere on the same file, he is no longer covering c6, and the
king slips through. If he doesn't move, the king captures him.