My car is a keyless ignition and as such just has a button I press to have it start. My question is... what happens if I hit it while I'm driving?
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It could work exactly the same as a keyed ignition, in which case your starter motor would engage, but manufacturers (and I think this is true for all) have added a simple piece of logic which only engages the starter if the engine is not already running.
This would be the ideal solution for keyed ignition as well, but it has only been in relatively recent years that engine control units have had the functionality, and there hasn't been much of a drive to implement it. Whereas with keyless ignition the stage was set to implement a wide range of improvements.
If you just push it, nothing. It would be rather hazardous if you could find yourself without engine power just by bumping a button.
If you hold it down for some time (e.g. 3 seconds on a Lexus (pg 117)), it'll shut off the engine, same as turning the key to off on a conventional ignition.
Early keyless ignitions vary by manufacturer, but most don't just let you turn the car off. Typically it's a 3-5 second hold. All the OEM keyless ignition buttons are software controlled, they aren't directly connected to the starter.
Due to recent "unintended acceleration" events and hoo-haa, a standard "3 second hold" has become the norm. I think even a panic detection algorithm has been added as well, when the button is pressed multiple times within a certain amount of time.
Custom keyless ignition systems on older cars are typically 'dumb', and just replace/duplicate the switch behind the ignition cylinder with a button. So they are directly connected to the starter and will attempt to engage it. At idle speed it might engage and slow the engine down, making slightly weird noises. At other speeds it's likely to fail to engage and make horrid grinding noises.
New cars however have abstracted the ignition button from the starter wire by many layers of software. Even more so after Toyota-gate.