Direct debit chargeback

As is well known, you can simply reverse a SPEA direct debit for up to 8 weeks without giving any reason. But what happens when the account that was debited doesn't have enough funds? So the money is now elsewhere. Does one of the banks involved have to step in first? Are there any mechanisms that initially block the money in the event of a chargeback? Are there any bank employees here who have experience with this? submitted by /u/Educational_Bee_62