I think you could describe it as she splayed her hands, as in
“What's the matter, Nick?”1 She blew out her breath, splayed her hands in front of her. She took a step closer to him and said, “I know what's going. I know." (Catherine Coulter, The Eleventh Hour, 2002)
She splayed her hands in exasperation. “What are you talking about?" (Michael A. Martin, Fallen Gods, 2012)
Quinn shrugged his shoulder and splayed his hands, “I guess that came out wrong." (Robin Wood, Murder She Spoke, 2014)
Of course splaying is just spreading, so one can splay one's hands on something, etc., which changes the meaning of the phrase, but without such specificity I picture a gesture something like this.
1 Incidentally, I'm sure it's a spurious correlation, but the name Nick and variations thereof co-occurred with this phrase in my search results to a startling degree.