It is a neat trick! Somehow those pieces of metal are storing a magnetic field like a permanent magnet, but release their "charge" when the field is broken.

There's another interesting effect where an electric charge can be pulled from that set-up. Lasersaber demonstrates that here: https://www.youtube.com/watch?v=S_ssUTRbRRs

I certainly can't explain it, and I don't think we have an adequate theory that does explain it (yet).