you can click the blue arrows surrounding the spin when you get that "rare spin" thing when you have all yellow dots.
it allows that specific reel to spin again while the others stay the same. and you can click each multiple times (i think the limit is 3 though [for each reel])
i really like this feature. the arena makes getting rares more time consuming, but the special reel thing makes you get the chance of getting a rare much better. but overall, getting rares is harder now (you have to go through either 6 easy matches or 3 hard matches to get a good chance at it)