As long as A.P.R. is attached, any attacks done to Knuckle by his opponent return aura to him, lowering the counter. Until all of the aura lent by Knuckle is returned to him, he cannot be physically damaged by his opponent. The reverse is also true; each of Knuckle's blows only lends aura to his opponent, causing no damage. The interest rate on A.P.R. is 10% of borrowed aura, accrued every ten seconds. As the amount of interest accumulates, A.P.R. also grows in size. If the aura borrowed from Knuckle exceeds his opponent's total aura, the opponent "goes bankrupt".
When this happens, A.P.R becomes I.R.S (Individual Ren Suppressor). Once I.R.S is attached, the person will be unable to use Nen for 30 days, as a way of paying back a loan. This is what happened to Gon, which made him unable to go to the NGL, due to the Chimera Ants being too dangerous. Not to mention
As long as A.P.R is attached to someone, it is indestructible until it's deactivated, when it will swell and explode. A.P.R will only accumulate interest when the user is within 50 meters of the target. When it's time for interest, A.P.R will notify the user with a message. That message is,
"It is time. You have accrued interest"
This could work against the user, as this could notify the enemy that the user is close and keep them on high alert.