Wear on an IGH by pre-selecting gear under power
Shimano IGH use springs to move cogs away from the cable pull, and what's happening is that lateral friction between the engaged cogs inside the hub is too great for the spring to overcome.

This is actually a good thing, because if one of those cogs slid sideways under power the load on the engaged remaining width of the cogs would steadily increase (same load, less engaged width) and very likely something would fail - you would chip the corner off one of the teeth on one of the cogs. Doing that isn't a problem, once. But over time you end up with a collection of hard chips of metal from the cogs floating round inside your hub. Some will inevitably end up on the wear surfaces of the cogs, or if they're especially large even wedging them so they don't rotate.

The reason you should try to avoid this is that you're not 100% reliable at keeping the cogs engaged until you back right off on the power. Every time it shifts just before there's no pressure on the pedals you run the risk of damaging something.