Absentmindedly Beating Their Breasts
There are people who absentmindedly beat their breasts when saying the Viduy without internalizing what they are saying. Their prayers may be compared to a scarecrow that is meant to chase away harmful birds. Once the birds realize that the scarecrow is a just a lifeless contraption, they swoop down to feast on the seeds in the garden.
Reciting the Viduy is meant to drive away the yetzer hara. Thinking that a person is sincerely remorseful when saying the Viduy, the yetzer hara backs off. But when it sees that the breast-beating is only an empty gesture, it comes back with a vengeance, strongly enticing the person to stray and transgress.
This may be the intent of the confession, "For the sin we have sinned before You through the speaking with the mouth," referring to the sin of saying Viduy only with our mouths, not with our hearts.