Copyleft is a concept and practice of copyright laws using by copyright holders to ensure the impossibility to restrict the right of any person to use, modify and distribute the original of the product and works derived from it. Any copyleft derivative should be distributed under the same license as the original product.
As far as Copyleft license is similar to Creative Commons Attribution-ShareAlike license (CC BY-SA), the Copyleft hardware represents an specializing concept based on the idea of open source hardware and involves the use of CC BY-SA license. Copyleft Hardware symbol was created to identify electronic products released as Open Source Hardware under a free license that identical or does not contradict to the CC BY-SA license.
Micro guide (SVG, PDF)
Eagle library copylefthardware.19.09.2011.lbr
By using the Copyleft Hardware symbol you confirm that you are bound with CC BY-SA license and permit to share, copy, modify and use your schematic solutions, firmware source code and device itself (including commercial usage) with the obligatory of authorship attribution, providing the source schematics, firmware source code* and all necessary technical specifications and data-sheets references in developer friendly form and keeping same license for all feature derivatives. By using the Copyleft Hardware symbol you confirm to share your firmware source code under compatible free license (e.g. GPL).
* As far as you are free to make commercial derivatives with this licence and in accord to this GPL commentary you must keep and share initial or modified firmware source code in separate open source library but you are free to leave commercial part of your firmware implementation closed.
Copyleft definition (Wikipedia)
What is Copyleft?
Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0)
Open-source hardware definition (Wikipedia)
Bebas Neue Font (tarball with ttf version)
"Import dxf polygons" Eagle plugin
Copyleft Hardware logotype by Dmitry Shalnov is licensed under a Creative Commons Attribution-NoDerivs 3.0 Unported License.