User blog comment:B214/Question and Answers 6/@comment-24098614-20170330202422/@comment-29963364-20170330211008

The King Piece increases your power from 10 - 100+ times your normal level.

I think it works kinda like a dial. You can "turn up" the dial to increase your power. But if you increase your power too much, you could kill yourself.

Ajuka banned the KP for three reasons (that we know of)

1.) Because it could kill the user if over-used.

2.) The user can gain an unfair advantage by artificially increasing their power.

3.) The high-ups of the Devil society (the Nobles) were using the KP to keep Noble Devils in the Top Ranks. They wanted to keep Lower-Class & Reincarnated Devils from advancing too high/far in Society.