I hadn’t heard of this thing and neither had 95% of the people in my office, so it’s news.

The eraser shield is just a very thing piece of aluminium with various holes cut in it. You stick it over your penciled work and use it to shield the bits you want to keep from the deadly eraser, while still allowing you to erase bits you didn’t want.

It took me ages to find this, but a good arts supply store should stock them.

If you’ve got the patience (I didn’t) here’s a video where one is apparently used.

Thanks to Dave O’Neill and Dave Stock for getting me onto this one.