Paul Tan was able to procure the patent itself and has published the filing on his website, paultan.org (or you can view it here through the U.S. Patent Office).
The short and sweet version involves the employment of a small cylinder encompassing the camshaft that is connected to the rocker shaft. As the drum rotates the rocker adjusts the valve lift and timing. There are several images and a statement of claims posted, which are well worth a look-see if you've got your engineering hat on.
[Source: U.S. Patent Office via PaulTan.org]