Yaffs is downloadable with a GNU General Public Licence v.2, as at http://www.gnu.org/licenses/gpl-2.0.html You get the source code, with all the benefits that brings. And of course, if you buy a commercial licence, you get the source code too, but without the encumberance of your responsibilities under the GNU GPL
Because it's Open Source, you can always see what the code is doing and trace through; and having the code means you'll never be in a situation where you are critically dependent on us, either from a technical or a business perspective.