There's a gotcha with this. You have to use an SDHC card. You cannot use and SDXC card. That means a more than 2GB and up to 32GB on the capacity. I just had a client with a 64GB card and it didn't work. Proxy and regular recording happen at the same time, which is cool. I feared you had to record to the main card and then select clips for proxy. Not so. You can record to both.

Good to point out, Phil...
Though at this point SDHC cards have become affordable enough that I keep a couple of extras in the camera bag and one in the Proxy SD slot -- and since recording times are so long even at the highest proxy bit rate, getting 8+ hours of proxy on a 32GB card means not needing to change cards all day...

Yup. Highest rate proxy is 443 minutes on a 32GB card.

Here is the page from the manual about the different proxy recording rates.

https://www.flickr.com/photos/omegabroadcast/16332628650/