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 KOT49H (4.4.2) 2013 N7: Stock KOT49H (4.4.2)
Touchpad: CM11 KOT49H (4.4.2) iTouch 5G: Stock 10B146 (6.1.2)