The Footwashing

          When Jesus announced to his disciples, "Ye are not all clean," they could not understand the specific uncleanness to which he referred.
           We understand, however, because the evangelist tells us, "For he knew who should betray him; therefore said he, Ye are not all clean." John 13:11
          And Judas understood.
          By concealing the meaning of the footwashing from the eleven, Jesus could tell Judas his fault between them alone - in accordance with his teaching.
          And by identifying the trespass with part of the foot, Jesus could convey his forgiveness to Judas through the symbolism of the footwashing.

To sum up:

The footwashing was about cleaning an unclean foot.

The foot was unclean because the heel had been lifted against Jesus.

The effect of the washing was to remove the uncleanness.

The footwashing symbolized the forgiveness of Judas for his trespass against Jesus.

The example that Jesus gave us is to forgive those who trespass against us.

As he has done to us, so should we do to one another.