PAP Question (When will the payment be taken from my account?)

Long story short, I have my parents on my account to ensure things get paid. I let them know, monthly, how much their bill is and they pay it through online banking bill payments. 

My first question is: if I sign up for pre-authorized payments - do they take the money out of my account on the due date or when the bill is sent to me?

If it’s withdrawn on the bill due date, my second question is: If my parents send their money at the beginning of the month, and the PAP is removed on the bill due date, will it only take the balance left oweing?

Thanks!