When faced with a very low backhand, you must change the swing path if you hope to hit a significant topspin. You can't just take the swing path up the back of the ball from behind the ball...you can't get the racquet below the ball.
But, similar to the 'buggy whip' forehand where the player sweeps from close to their body and exaggerate brushing out away from them, you can hit a lot of topspin (and a little side spin) on a ball that is only inches off the ground. Drop your head of your racquet down by your left ankle (if your right handed) and brush up and out well away from you with a very closed stance.
This is a shot that must be practiced and perfected. Otherwise, you can always hit an effective slice on virtually any low ball. (As others have mentioned.)
Author, Tennis Mastery & Coaching Mastery