Negative policy rates pull the entire front of the curve below zero, and at the extreme a government can be paid to borrow. Investors accept it because a small guaranteed loss can beat the cost and risk of storing cash physically.
The trade-off is bank profitability. Lenders rarely pass negative rates on to retail depositors, so the margin is squeezed and the credit channel can work against the policy intent. Most users introduced tiering so that only reserves above a threshold were charged.
Example: a 2-year note yields minus 0.60%. Buying 100 of face today costs 101.21 and returns 100 at maturity, a guaranteed loss of 1.21 per 100 held to the end.
Related: yield-curve-control, quantitative-easing, target-range, bank-reserves