When playing the guitar, a movement of the pick toward the body of the player, thus away from the ground. It is easier to do if you turn the pick so that it is perpendicular to the strings or even angled somewhat towards the ground.

Up"stroke` (?), n.

An upward stroke, especially the stroke, or line, made by a writing instrument when moving upward, or from the body of the writer, or a line corresponding to the part of a letter thus made.

Some upstroke of an Alpha and Omega. Mrs. Browning.

© Webster 1913.