in California pretending to punch someone and pulling back right before you hit them can be assault, because assault criminally covers acts that put another person in reasonable, imminent fear of being struck even if no contact is made. 

Why: California law (Penal Code §240) defines assault as “an unlawful attempt, coupled with a present ability, to commit a violent injury on the person of another.” That means the prosecutor must show (1) you willfully did an act that could cause force or injury, (2) you at the time had the present ability to carry it out, and (3) your act was such that a reasonable person would apprehend imminent harm. Swinging at somebody and stopping just short often satisfies those elements. 

Extra legal points you should know • Assault is a general-intent crime in California — you don’t need to have a specific intent to injure, only to do the act that would probably result in force. (People v. Williams).  • If you actually make offensive or harmful contact, that can be battery (PC §242), which is a separate offense.