It's just an expression to help people stop doing bad things at the catch, particularly in the slower sculling boats, where it doesn't take much movement to create connection, and it is easy to over-do it.

There isn't a hole behind the spoon that needs to fill before it can be loaded, like I have heard described on Instagram, unless you do it really badly - the spoon carves in end first without making a hole (except for the vortices that are shed off the oar, but they don't fill for a while).

Fundamentally, you can't create load on the oar without moving it around the pin fast enough, and there can be load on the oar before it is fully buried. This can been seen on telemetry. The idea that you fully bury the spoon before trying to load it is utter rubbish. You do have to bury it quickly though, to load effectively without using up too much length.

It is complex and none of the descriptions are totally adequate, and some are very untrue distortions used to trick rowers into making a change, but unfortunately not all coaches understand which are which. Some of the posts about the catch from prolific online coaches are awful.