Underhanded as an adjective, meaning “deceitful, sly,” as in He played an underhanded trick on his parents, and “deceitfully, slyly” as an adverb, as in He did it underhanded, is Standard in both uses. Underhanded is a flat adverb, and underhandedly is an exact synonym, also Standard: He did it underhandedly.
Underhand is also both adjective and flat adverb, but although it can mean “deceitful,” its more frequent meaning is a literal description of a throwing motion made with the arm always below the shoulder and contrasting with an overhand throwing motion: She made an underhand throw. She threw underhand to first. Underhanded is also Standard in these throwing senses, as both adjective and flat adverb, but it is somewhat more common meaning “deceitful” or “cheating”: It was a dirty, underhanded trick she played.
