I haven't seen much that really does a good job explaining it, so even my understanding is mostly intuitive.

Basically, as I understand it, SteemPower doesn't really exist. The blockchain tracks staked tokens in terms of VESTS and SteemPower is just an accounting trick to make it easier for people to think about.

As an example, it's conceptually similar to a mapping of centimeters to kilometers, with the difference that the blockchain is constantly changing the conversion factor when it produces new VESTS and SBDs, so any time you want to do the conversion, you have to find out what the current ratio is.

FWIW, here's a description of how to convert from VESTS to SP: Converting VESTS to STEEM.