Her bum, when impacting with an object, causes it to vibrate at it's resonant frequency, sustained contact or large impact causes it to shatter. Or maybe she just hits it really hard It's like she's dancing to some background music, unaware her flank is impacting with objects causing them to break. By the end of the song half of Ponyville will be destroyed Nice you painted happy Derpy, seems all the art I see of her nowadays is her looking sad, good to see her smile again
All hair your artwork and the mare with the iron plot
Derpy has the softest most destructive flank in all of ponydom. I imagine the force of her plopping her butt on the ground from that one scene as being equivalent to having dropped a construction beam from a hundred story height, yet her natural bubble butt seems to absorb 99.8% of all shocks leaving derpy to feel almost nothing when something hits (or in this case she hits) her buttocks. Truly a marvel talent; being able to repel all forms of danger/attacks with just ones flanks/bubbly butt with nearly any effort.
Take heed all who follow Derpy and be sure to give her a good 10-20 feet of space if behind her, she can make sudden stops without warning!