This is kinda cute. But I thought Tenshi would look like she's enjoying it more, or that the prinny would explode. The Flan doesn't make the screen explode :(
Charge is accumulated by catching the rock while it falls. It seems to add a multiplier to how high you can toss the rock.
You can also reach rather high tosses by holding the rock at low altitudes, and opening up the right-click menu. Then, click off of the flash (closing the menu) and bring your mouse back into the flash area from the top. If done correctly, this should catapult the rock.
Combining these strategies, you should be able to breach the 9000 mark. As for the 9 digit levels, I could only achieve that by hacking my charge up to an arbitrary high number.