I haven't properly checked this on the Auris, but on other cars (e.g. VW), if you exceed the cruise control speed on the throttle for over a set time (5 minutes?), then the cruise disengages. Also, if the car drops below 25 mph on cruise then it will disengage it. It seems to vary by make and model as to what a 'dab' on the increase control does; on the Auris it is a small increase, VW is the same, the Mazda we had, incremented the speed by 1 mph, so, say, 5 quick control presses would gradually bring the speed up by exactly 5mph.