I'm not a dev. But the new RC system seems to work a lot like Voting Power. The short answer would be that powering up affect the RC regeneration rate. More SP you have, more RC you get each day.
This is actually one advantage the old bandwidth model had. You could just power up and use more of the network. But Resource Credits are kind of like another token that is non-transferable. So RC is no longer a claim on a portion of the network.
The amount of RC required for a certain operation is decided by the network and we are still waiting for the equilibration. RC is basically just a token you pay (burn) which has a max limit and regeneration rate based on your SP.
RC has more in common with rshares than Bandwidth.
Hope this helped.
Update: The new 0.20.4 fixed some issues:
#2961 Powering up / delegating SP does not increase RC
Bug: users powering up, were not able to vote/transact instantly
Now, receiving SP gives an instant voting mana boost