Supple sidewalls.

When you sit on your bike you flatten the contact patch with the ground. As you ride, the contact patch is constantly moving around the tire. Try to flatten a section of tire with your finger. The energy that's required to constantly change that flat contact patch has to come from somewhere. That's rolling resistance.

More air pressure reduces the size of the contact patch so it reduces the rolling resistance - to a point. A rock hard tire, however, bounces upward over every pebble on less than perfect roads. That often feels faster but it really isn't. The energy that's required to lift the bike vertically isn't moving you down the road and has to come from somewhere.

A tire that has very supple sidewalls requires less energy to deform at the contact patch and envelopes small road irregularities better so, at an equivlent air pressure, it will roll more easily. The trade-off is likely to be durability. A tire with very thin, supple sidewalls is likely to cut more easily.