In most years, Social Security’s COLA results in a large enough increase that higher Part B premiums can be deducted from the new payments without ending up with Social Security checks that are lower than the previous year.
For beneficiaries who are “held harmless,” and for whom the Part B increase would otherwise exceed their Social Security COLA, the Part B increase ends up being limited to the amount of the COLA. That means their full COLA is used to cover the Part B premium increase and they’ll end up with Social Security checks that are exactly the same as they were the year before.