From a spin jump, a shinespark wind-up is triggered by releasing either forward or jump (or both) and pressing either up or angle-up.
The wind-up is a brief period during which the player can control the direction of the spark.
During the wind-up, if jump is pressed together with up, angle-up, or forward, Samus will immediately spark up, diagonally, or horizontally, respectively.
If the wind-up expires, Samus will default to a vertical spark.
If jump is held when activating the wind-up by pressing up or angle-up, then the wind-up is effectively skipped as the spark activates immediately.
See also: Tech Page