"Throw rock first" is a common RPS psychological strat.

They were saying that Chisato will always go for the tie that first strategy results in, and then use that as the basis to immediately detect an individual's hand differences when they're about to change. She knows what your rock looks like now...or will tell with her super reflexes that you're changing.

She eliminates the need of ever having to throw rock herself after that very first round, so she will always tie or win thereafter.

You only have a chance of beating Chisato on the first throw, before she gets a sense of your attire and muscle movements to invoke her own unbeatable strategy.