On my new build I will be using a tether kill switch that you would typically see on an ATV, this is the one I picked up -
http://www.amazon.com/gp/product/B00378K6L0?keywords=Pro%20Armor%20A040021%20Black%20Universal%20Kill%20Switch&qid=1455686419&ref_=sr_1_1&sr=8-1
These are a "Grounding" type switch, when the clip is pulled the switch inside connects to chassis ground. Utilizing a standard relay will allow this type of switch to work in an automotive 12v environment, I have bench tested my setup and it works very well. Mine will be installed so that it will cut power to the entire vehicle by disengaging the continuous duty solenoid that controls all power from the battery. It could also be installed to cut power to the fuel pump and some part of the ignition.
The reason I wanted this type of "kill" switch is the ability to attach the lanyard to either myself or to a stationary point between the driver and passenger seats. Instead of reaching for a button to push I will only need to "sweep" my arm across the lanyard which will give a much larger target to hit in a panic.
I also intend on installing an oil pressure switch (the style that would turn on an "idiot" light) that will also act as a kill switch if I ever lose oil pressure, ie. rollover or mechanical failure.