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Liquidation is a process of reducing a bad debt risk. Whenever a Health Factor of a user falls below 1, it means the user hasn't managed her/his collateral & debt properly and her/his position is unacceptably risky. In such a situation protocol allows anyone to repay the user's debt and take his collateral. The value of collateral taken is equal to the repaid debt value + liquidation penalty fee.
To become a liquidator you need to observe the state of the protocol. Whenever you find that the Health Factor of any user is below one you can perform a liquidation by calling the "liquidate" function.