It's not a technique, as such. ATC stands for Artist's Trading Card. They are always the size of a traditional playing card - 2 1/2 by 3 1/2, and can only be traded, not sold. You can decorate them in any way you choose, though when participating in swaps there may be restrictions.
Loads of information here -
http://www.cedarseed.com/air/atc.html, and I see it's even been updated recently.
Hope your DH is doing OK.