Ura-tsuki (literally: “inverted/hidden-fist thrust”) is the karate equivalent of a boxer’s uppercut. This is essentially a vertical version of a hook punch, which is used to strike an opponent’s solar plexus, or the underside of their chin. Uppercuts are extremely efficient, powerful techniques. However, the uppercut’s extremely limited range means that it can only be used in the clinch.
Uppercuts must start out like any other punch, to prevent you from telegraphing. Then, twist your hips towards your opponent, and drive your elbow towards and slightly-up your centerline, much like a forward elbow strike. Picturing that your elbow pushes your fist result in a significantly faster technique than imagining that your fist leads and your arm follows. Your fist does not “turn over," to ensure that you strike the opponent with your knuckles, and not your fingers. Simultaneously pull your opposite-side hand to its hip, for reciprocal action.
[Video of uppercuts fast and slow, from the front and side.]
Because uppercuts operate over a short range with no turn-over, they need a significant drive from your hips, legs, and core. Uppercuts can deliver incredible power, but only with incredibly efficient biomechanics. As such, they are typically reserved for advanced students.
Uppercuts have the added advantage of being difficult to defend against, since they act quickly, and just outside of the opponent’s field-of-view. This is why ura-tsuki can be alternately translated as a “hidden punch.”