A forehand hit from the backhand side of the court, directed down the line to the opponent's forehand side. The player runs around their backhand (like an inside-out forehand) but hits the ball down the line instead of crosscourt. It's the riskier sibling of the inside-out forehand because you're hitting to a shorter part of the court, but it catches opponents off guard when they're expecting the crosscourt angle.
"He runs around the backhand and goes inside-in down the line. Nobody was expecting that. Winner."