In the Penal Colony
How we cite our quotes:
"He doesn't know the sentence that has been passed on him?" "No," said the officer again, pausing a moment as if to let the explorer elaborate his question, and then said: "there would be no point in telling him. He'll learn it on his body." (12)
The penal colony's use of the apparatus is based on this idea that the prisoner somehow learns something through his body and its wounds that he does not learn by merely being told. It's as if real learning, or understanding, can only come through suffering – it can't just be intellectual.
[The officer:] "Of course, the script can't be a simple one; it's not supposed to kill a man straight off, but only after an interval of, on an average, twelve hours; the turning point is reckoned to come at the sixth hour." (16)
Merely killing or punishing is not the point of the apparatus. The suffering the condemned man undergoes is supposed to bring about a transformation in him, and this requires that it last for quite a while. The long length of time calls to mind numerous elaborate ritual sacrifices or punishments, not the least of which would be crucifixion (if you wanted to read this as a religious allegory).
[The officer:] "The first six hours the condemned man stays alive almost as before, he suffers only pain. After two hours the felt gag is taken away, for he has no longer strength to scream […] Only about the sixth hour does the man lose all desire to eat." (16)
In the process of suffering under the Harrow, the prisoner becomes incredibly weak and basically loses his vital functions, his ties to life. In the last six hours, his attention can be concentrated entirely on what is being painfully inscribed on his body – his concentration will be entirely on his suffering. That's the point.