Point your right thumb along the purple DNA helix at the top of the article (either direction). Your right fingers curl in the direction the helix turns, so the purple helix is right-handed.

Now do the same with your left hand and the orange DNA helices. When your left thumb points along the helix, your left fingers curl in the direction the helix turns, so the orange helices are left-handed.

Make a fist with your right hand. Which way do your fingers curl?

Thank you for explaining the convention (I've heard it before). How does that relate to "righthandedness" otherwise?

I was just (jokingly) pointing out how introducing conventions for right or left requires memorization of some silly rule like this. I'd much prefer if they used made-for-purpose topological terms instead of pretending how this somehow relates to a widely known concept.