Interesting. Although having fiddled and driven (round the yard) with Dexta's I know very little about them, it seems that the glow plug with fuel tap is the predecessor to the commonly found thermostart, which is a unit containing a 12v coil and fuel supply. After a few seconds of power the unit gets hot enough to open a valve to allow diesel (from a reservoir in the leakoff system) to the coil which burns and provides the heat to aid starting. It's highly effective and as it all works by just having power applied for 20-30 seconds from the heat position of an ignition switch it requires almost nothing in the way of effort* from the operator. *I do know at least one tractor operator (he's really a cowman) who has so little patience and sense that holding the key to just before 'start' for a full 30 seconds would be a challenge. He'd probably count to about five then open the throttle to full and just keep cranking!