You are referring to the backdoor Roth IRA, which has a contribution limit of ~$6k/year. A Mega-backdoor Roth is an even more extreme tax loophole, specificall

You are referring to the backdoor Roth IRA, which has a contribution limit of ~$6k/year. A Mega-backdoor Roth is an even more extreme tax loophole, specifically involving employer 401k plans, which has a similar taxation structure just managed by your employer, and offers $30-40k of additional tax advantaged opportunity.