Speedlite 600EX-RT and the ST-E3-RT provide a remote shutter release
New in the Speedlite 600EX-RT and the ST-E3-RT is Linked Shot, this lets a photographer trigger the camera shutter release by pressing a button on the flash. In effect the flash becomes the radio trigger for the camera’s shutter. You may think it’s a gimmick but I’ve found it to be more useful than I expected.

For this picture of Alexa I positioned a Speedlite 600EX-RT behind her in group B with the zoom set to 35mm to ensure I would get the rim light. The background bushes behind her are lit by the mid-afternoon sun and blown out intentionally. To be honest the camera was right up to it’s sync speed of 1/200s. Alexa was stood in the shade of the building and so I used another Speedlite 600EX-RT with a small Chimera softbox in group A, to add some light on her face. The ST-E3-RT on the EOS 5D Mark III was set to E-TTL II for group A and manual for group B, the rim light. I put the camera on a tripod, but then realised I had not got a second light stand to hand. I used the Linked Shotfeature to trigger the camera from the group A flash with softbox and I held this in my hand. In effect I was both photographer and voice activated lightstand (VAL) combined
Configuring linked shooting on the Speedlite 600EX-RT and ST-E3-RT

Hidden deep in the Speedlite 600EX-RT Personal Functions P.Fn is where you find and activate Linked Shot. (P.Fn 07) I’ve found that I have this active on all my Speedlite 600EX-RT flashes so it’s just ready to use whenever I need it.
Access to the flash personal functions is only possible from the flash not the camera menus. Press and hold the Speedlite’s Zm/C.Fn button until the Custom Function menu shows then release it and press it again to access the personal functions.
March 2013 – update. There’s an error in that Linked Shooting and Remote Release from the flash to the camera are different. Linked shooting is somewhat different in that it allows a camera with an ST-E3-RT or 600EX-RT to trigger other cameras. P.Fn 07 then determines if a Speedlite 600EX-RT will also fire it’s flash as well as trigger the camera. Make sure to check out the more recent post on using Linked Shooting on the site.
The ST-E3-RT will communicate directly to the EOS 5D Mark III and EOS-1D X through the hotshoe, no cables needed. For other cameras there’s a small connection port on the side of the ST-E3-RT and the Speedlite 600EX-RT that you connect with an optional cable Release Cable SR-N3 to the camera’s remote release port.
There’s one important point to remember, tethered shooting with Live View mode deactivates the Linked Shot function
Canon has used the term linked shooting before to allow certain cameras with WiFi transmitters fire in sync with a master camera. The Speedlite 270EX II and Speedlite 320EX also support linked shooting, but only using the infra-red remote trigger.

“Hidden deep in the Speedlite 600EX-RT Personal Functions P.Fn is where you find and activate Linked Shot. (P.Fn 07) I’ve found that I have this active on all my Speedlite 600EX-RT flashes so it’s just ready to use whenever I need it.”
To clarify…
You can use/access linked shot whether this personal function is on OR off.
P. Fn 07 determines whether the flash fires during linked shooting.
Andrew
Indeed Andrew you are right, in fact I also see that there is a need for several other areas to be clarified in this post.
Particularly the linked shot feature is actually not the same as the remote camera release or flash test firing. This is available from all slaves in radio mode.
Linked shot allows other cameras to be triggered over radio links from a master camera, there’s a more detailed look at the feature in a recent post here: http://www.p4pictures.com/2013/02/capture-behind-the-scenes-as-you-shoot-with-linked-shot/
thanks for the comment
Brian / p4pictures