After one finishes his prayer with the goodwill from his Master (that is, after one says, “May it be Your goodwill to accept it”), one needs to imagine that he is giving up his life, with perfect love – and that is called “Tachanun”, or “Supplication.”
Once he does this, he is forgiven all the mistakes he had made, and if he did things that are not befitting a Jew, and did them in public – which amounts to the profanation of the Name of God – even that is forgiven. That is why initially it was called “Falling on one’s face on the ground” – because ground and land are connected to Rulership, the lowest spiritual level, from which there is no way but up.
He thus falls into the kingdom of Evil, with the goal of finding the small particles of goodness there, to bring them up when he straightens. He then goes together with them to the level of Rulership again, but this time in the highest world of Nearness.
The source for this is the Zohar at the top of