The common advice that smooth is fast is just plain wrong. It’s the worst advice you can give to a beginner… totally useless and, in fact, misleading.
The idea that it is necessary to be smooth to be fast is probably an illusion, possibly caused by looking at a car driven by an expert from the outside. It ignores what’s happening on the inside.
From inside the car, the driver should be making constant, rapid and sometimes even violent corrections. If not, the car is not being driven on the limit. The expert autocross driver doesn’t look smooth from an in-car video or to a passenger nor does it necessarily feel smooth to the driver himself. After a 60-second run the driver may well be huffing and puffing from the exertion and from having his breathing interrupted by high lateral-G forces.
Now, if the car looks herky-jerky from the outside, then the tires are probably being shock-loaded beyond their capacity, grip is being sacrificed and the positioning is not correct. All such faults are most likely the result of not looking ahead, not due to a lack of smoothness. So, if looking from the outside is what people mean by “smooth is fast” well, okay, but it’s so misleading as to be useless. To look smooth from the outside, and therefore to be FAST, look ahead. Do not “try” to be smooth. There is no more colossal waste of time in autocross than “trying” to be smooth. It can only slow your reflexes, make you late and make you fail to transition rapidly enough. All things that make you SLOW.
All that said, it is true that we do get smoother as our car control skills improve, as we look farther ahead and as we better anticipate what the car is about to do next. That type of smoothness only comes with experience. Forget about it. Learn to drive on the limit, smooth or not, and you’ll get fast. You can be smooth in your old age.