Back up preparer in ARM cannot pick up reconciliation to prepare it

Hi,

I cannot find any information on ARM back up preparer and how it works in the admin or user guide.  I have some users set up as back up preparers and they would like to pick up the rec to perform the work but can't do so.  they can see the rec but cannot create a lie in explained balance...etc

Is there any special setting that needs to be done apart from setting them up as backup preparer? Many thanks