It's triggered by bridging some pins on the Dell 30 pin connector.
I dont know which pins they are (as the connector is mostly undocumented, or just as much the 3rd party connector makers havnt documented it) but the pins might be for that specific purpose.
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Nexus 4: Stock JWR66Y (4.3) 2013 N7: Stock JSS15R (4.3)
Touchpad: CM10.2 JLS36I (4.3.1) iTouch 5G: Stock 10B146 (6.1.2)