Rollover from IRA to 401k: transferring IRA (traditional, Rollover, SEP, SIMPLE) to an active 401k. This is known as a Reverse Rollover.
Note that after-tax IRA and Roth IRA cannot be transferred to traditional or Roth 401k. In other words, a Reverse Rollover always assumes transferring pre-tax only money from an IRA to a traditional 401k.
When leaving a job, usually, it’s advised to take your 401k (i.e., transfer it to a Rollover IRA). So why would you consider reversing the transfer from an IRA to a 401k? Let’s explore the cases where this type of Rollover can be helpful.