Hi Lord Harris,
You asked:
Would anyone care to give me a brief description of this and when it would be of main use?

The overhand right (or left) is a rear-handed slightly-hooking, downward convex-arc punch normally used against a matching-stance opponent (i.e. orthodox vs orthodox or southpaw vs southpaw) that is mainly intended as a simultaneous counter over an opponent's left (or right) jab.

More descriptively, it is thrown like this from a left lead (i.e. orthodox) stance:
1. "Trigger step" 30 to 45 degrees to the left with your left foot to open up the hips and generate momemtum.
2. "Hip-whip" (as you would, for example, when you swing a baseball bat) the right hip towards your opponent to recoil the right (striking side) shoulder and put hip & leg torque into the punch.
3. Raise your right elbow slightly above your right shoulder and turn your right fist at a 30 to 90 degree position (you will naturally adjust your fist position according to the relative position of your opponent's left arm as you counter) with the right palm facing away from you.
4. As you do this, lean/angle your body to the left (your weight distribution now primarily shifting to/over the lead left foot) and "shoulder-whip" your right arm "rainbow-arching" it over your opponent's left arm as you shoot it to and through the left side of your opponent's jawline.
5. Follow-through with your shot as you strike through the jawline target. Simultaneously, let your rear right leg naturally follow-through, "gliding" forward (right knee slightly bent naturally) as your punch makes contact.
6. Retract your punching hand back in a "pulling" arc back to your guard position.
7. As you are doing the above, be sure to have your chin tucked and left hand up protecting the left side of your head as you are throwing your overhand punch.

This punch is a versatile one, as all boxing punches are, that can be used to discombobulate an opponent with its non-traditional oblique trajectory. Besides the aforementioned, you can use it to split an opponent's high guard and nail him down the middle. It is also a potent punch that a smaller man can effectively employ against a larger/taller adversary, especially at close range. Furthermore, it is an effective punch to use in combination with the other more "traditional" boxing blows to confuse and overwhelm opponents who are used to defending the standard lines of attack.

Anyway, I hope this helps you out. Take care...