[smeej]

Your thumbs run vertically along the sides of the card, parallel to each other. You use them to cover anything to the sides of the month header(/footer) you're looking at. I think it's that "not under or above" description from the link that's confusing.

Find the header/footer for the right month. Use your thumbs to cover everything to the right or left.

[billbrown]

Doesn't a thumb then obscure the label of the month you're identifying?

[smeej]

Not if you put them to the right and left of the month's label. That's the whole point. The thumbs cover everything to the right and left of the label.