Board Thread:Speculation House/@comment-5356197-20141028085117

While she hasn't made obvious use of any Semblance; it's also true that several characters don't actually have to. Her "powers" section speculates that she has great strength; and I would hazard to say that that IS her Semblance. Like Thor, she thrills at a challange, a test of her strength, "we'll break his legs."

You can also hear a tone singing right before she does a powerful swing. A pitch lower than the one Weiss has when using her Glyphs. Ruby protects Weiss from one such blow during the food fight. Which leads me to speculate on two other things...

Like Ruby's petals, the "tone" from people's Semblance may actually be audible to the characters in question. Ruby having speed may allow her to react to said tone.

Ruby has learned the depth of her teammates Auras, and Weiss may well be the weakest of the team (while still being the most valuable teammate). Ruby's abundant aura would allow her to recover faster than Weiss; just not fast enough to do it again, at that time. 