if you have a loan of 1m at 1% interest (yes, that was the interest in Denmark before covid), then you can sell that loan for 800k and refinance with a new loan of 5% interest.

in turn you cut off 200k of your loan.

This technique obviously only works under the assumption that the interest rates are going to drop long term, so you can refinance back to the lower rate later (which all of the people who made the trick believes).