Confession and Penance

While reading the Catechism of the Catholic Church I came across this passage,

The confessor is not the master of God’s forgiveness, but its servant. […] He must pray and do penance for his penitent, entrusting him to the Lord’s mercy.

Catechism of the Catholic Church 1466

What struck me is the mention that the confessor (priest for example) will also do penance for those that confess to him. I’ve never even thought that this was something the confessor was taking on to himself until now.