Camera Remotes
Most handheld HDSLR setups make it impractical to start and stop recording because the buttons are on the back of the camera and the operator’s hands will generally be positioned elsewhere gripping handles on a support or tripod. That's not to say it isn't possible; it's just not easy. Some SLR remotes (not all) can control the start/stop function of the camera in Live/Video Mode.
Software Control
Many HDSLRs can be remotely controlled through a computer. The camera is connected to a computer via a USB cable; software can display the live view and control most of the camera's features. This opens up a whole list of possibilities when the operator needs to be at a distance from the camera, but still see and control the image. The camera control software is included with the Canon HDSLRs, and Nikon offers it as an option for about $145.
The most important controls that can be accessed from the software are the exposure controls (except for Aperture on manual lenses), picture style/control settings, focus, and start-and-stop operation of video and stills. The live view image can also be displayed in the software.
The video can be recorded to both the card and the computer directly, or just to one or the other. One of the drawbacks is that the live view image is not displayed at its full frame rate. This choppiness means the USB setup cannot replace a proper monitor. The computer option can still be useful in situations where it's acceptable to view the video at a lowered framed rate (note: this does not affect the frame rate that's being recorded). To make matters worse, though, the HDMI and A/V outputs of the camera are turned off when the camera is controlled via USB, so there is no proper way to monitor the video when using the control software.
Focusing is also limited because of the choppy and delayed video, which is another reason this will be useful only to some users in certain situations.
Still, this can be an interesting option. If the operator doesn’t want to carry a laptop computer, then even a tablet computer or an iPhone or iPod Touch with the onOne app can be used.