Je mehr Pferde du anspannst, desto rascher gehts - nämlich nicht das Ausreißen des Blocks aus dem Fundament, was unmöglich ist, aber das Zerreißen der Riemen und damit die leere fröhliche Fahrt.
The more horses you harness to the job, the faster the thing goes -- that is to say, not tearing the block out of its base, which is impossible, but tearing the straps to shreds, and as a result the weightless merry journey. [Kaiser/Wilkins]
The more horses you put to, the faster your progress -- not of course in the removal of the cornerstone from the foundations, which is impossible, but in the tearing of the harness, and your resultant riding cheerfully off into space. [Hofmann]
The more you strain to move the block, the faster you'll go when the straps break. So more force means more speed, but not more effectiveness.
It's comical to think of someone gaily zooming along, thinking he's dragging the block behind him all the while. What you really want here: that's the question. If you want to move the block, that's impossible, so why try? Only to prove impossibility? If you want to fly, why bother with the block? The desire, then, must be to be released from the block, to feel the maximum effort has been made. Wouldn't that be the same as achieving the point of no return? Having made the greatest possible effort, you are now free. The only question then is, whether or not you have made the greatest possible effort, or if you might be able to do more. How much is enough?
What is the block holding up? Why are you trying to pull it down? If the block is only an abstraction representing any arduous task, then the aphorism is more or less saying that the harder you try, the sooner you'll be done, one way or the other.
The addressee is the informal "you," so I imagine Kafka saying this to himself. You keep making these supreme efforts, he seems to be saying, but is that really because you want to succeed, or is it because you want to break down and be done with it finally? In that case, wouldn't the more correct course of action call for less effort rather than more? Even though the task is impossible anyway? Or is it that you need to think of effort differently, not in terms of greater force, more struggle, but some other way? Perhaps the greater effort is not made by pulling harder, but by paying more attention, and finding the right route?