>>1681002fuel canisters are a mix of butane/isobutane (~80%) and a small amount of propane (~20%).
Butane vaporises at 30F, isobutane at 10F, and propane at -44F.
When it gets cold enough, the butane/isobutane will stop vaporising. With an upright stove, only the propane will be able to flow upwards and into the burner. But once your canister runs out of propane (which is only 20% of your fuel), your stove will stop working, leaving you with 80% useless fuel.
With an inverted canister stove, you can flip the canister, so the propane floats to the back of the canister, and creates pressure to force the butane through the burner. This allows you to burn the butane in your fuel canister first, so you can actually use all the fuel you're carrying.
https://www.youtube.com/watch?v=vbuhwHfhR18 