I'll try to explain it.
So, you have a fully-charged O-power that you want to use, let's say it's Hatching Power. You have this nice Hatching Power sitting there, just got it, it's new and sparkly and fully-charged. But, you have a backlog of eggs to hatch. "Well," you say in my voice because I don't know what you sound like, "I'll just use the Hatching Power and then save-scum to recharge it!"
So, you go to use the Hatching Power. "But wait," you say, "If I use it now, then save, it won't recharge with save-scumming!"
Then, you have an idea.
"I know! I'll save right now, BEFORE I do anything!"
You save the game, then you select the Hatching Power. You go for a bike ride for three minutes. The Hatching Power wears off.
"Now, I'll soft-reset without saving."
You close the game, then open it again.
"I should see if my Hatching Power is charged, yet..."
So, you go and look at it. Lo and behold, it is fully-charged because you had saved BEFORE you even touched it! Granted, that egg you just hatched before closing the game will need to be hatched again, but hey! You can still level up your Hatching Power by save-scumming like this! Thus, all is right in the world.
Happily, you spend the next hour and a half repeating the process twenty-nine more times.
"Awesome! It leveled up! And all I had to do was save-scum!"
You decide to hatch an egg or two, then do another save-scum session like the first one once the Hatching Power charges up. At this rate, you'll have a fully-leveled Hatching Power in about a day! Awesome!