the nature of sin

[BrianSoddingBoru4]

It isn’t. The prayers are voluntary. A Catholic priest has no authority to punish a sinner.

Boru

upon receiving confession isn't it the job of the priest to offer contrition or pentance for the sins confessed?

Bucky when he was a catholic school boy:
Bless me father for i have sinned, it has be 15 hours since my last confession.

Preist: what could you have possible do in 15 hours?

Bucky: I viewed and masturbated to gay donkey porn, then fashioned a large donkey 'wood shop project' out of a tree branch and reenacted my fav scenes. 

Priest: 
bless you son for the confession of your sin your penitence shall be 10 our fathers 36 hail marries and burn your wood shop project.

here used the prayer is used as a punishment/way to pay for your sin.
pentance:
1 : an act of self-abasement, mortification (see mortification sense 3), or devotion performed to show sorrow or repentance for sin He did charitable work as a penance.

Your definition is correct - it is an act of ‘self’ abasement, and is therefore voluntary. ALL acts of penance are voluntary. The prayers usually said after confession are the outward expression of the inner feeling, hence the term ‘act of contrition’. Contrition is what you feel, penance is how you express it. 

But prayers are never ‘used as a punishment’, that’s just silly. It’s simply more convenient for both priest and penitent if the act of contrition takes the form of prayer. 

Boru