You do have to hold the wheel, precisely because you might find yourself having to assess what the car is doing "in a split second", and possibly issue corrective actions. You can't do that unless you are holding the wheel and are paying attention to what the car is doing! But all that means is that the car is not really driving itself; it is however dealing with the boring, predictable parts of the job and leaving the rest up to you - this makes it easier to be attentive, not harder. The tasks where it's hard to pay continued attention are ones where you have to do something that seems extremely predictable, but also has very rare events where it isn't. Computer assistance can actually help a lot with such cases.