It means that you validators use PoW algorithms to hold their stake (like in Ethereum 2.0) and/or write the state of the PoS blockchain on a PoW blockchain once in a while to prevent long range attacks (long range attacks = when someone steal all the keys of the validators in the genesis block, and create and equivalently long PoS with hardly any effort)

You need to quantify what that means for energy usage, as that is the main point of contention. Fossil fuels are the primary contributor to global warming as they're used now, but no one should have any problem with, say, a portable gas generator used during an occasional blackout.

You explained how this applies to ETH 2.0, how about Cardano and Polkadot?