curious thing: Bananamatic mentioned the possibility that bullets are slowed down (perhaps instantly, or frozen and then move again as they unfreeze), a possibility which I haven't thought of, initially (because I haven't played Guwange, or Mushihimesama Futari's Arrange Mode..). Rather, I thought that the gameplay would be a bit Progear-like, where converting bullets into jewels would make bullets near the converted bullets to be converted as well, and then the next bullets near these converted bullets (Diamond waves? In MY Touhou?).
My guess (which may be wrong, but guessing is fun, when the answers are sure to come; pointless, otherwise..), is that shooting will raise a gauge or level, which you spend when you want to freeze patterns and start a chain. If the game's like this, then you have Aya's defensive abilities.. on steroids! (everything's better with steroids, right?) ..erm.. think of some bullet-eating mechanic like in Giga Wing/Ikaruga, where you collide with bullets you're "supposed" to be avoiding. Though I wish that the game would feature partial bullet slowdown (as a defensive resource), or Cirno's ability to scatter shots to counter bullet walls or to avoid being cornered ( Cirno reduced the amount of shots whenever she froze the screen in PoFV, besides turning some of these shots into chainable shots, maybe to compensate for the random trajectory change of said frozen bullets.. just as an example)
