Turn 12 (Kink)
Exiting Riverside, get back to the middle to right side of the road to set up for the Kink. This is a fast corner with a bump just after the apex that can be unnerving. A brief lift in the throttle right before turn-in then back to maintenance throttle squeezing to full throttle through the corner keeps weight on the rear of the car and generates a comfortable, stable understeer through the bump.
Turn 13 (Lost Hill/Phil Hill/Magic Mountain)
This corner is similar to turn 5 (Cotton Corners) as it is another blind corner with a late apex that is out of sight until you get there. Set up for this corner by being on the left side of the road, brake in a straight line and finish all downshifts before turning in. Trail off the brakes and turn-in early to make sure you get down to the apex. Get back to power as you are cresting the hill. Emphasize a later apex to the point where it looks like you are going to go off the track to the right. I use the apex curbing a little, it launches the car in the air a bit, but if you had a good turn-in point then you should land for a perfect track-out.
*The most common error for this corner is turning in too late and trying to square-off the entry. You want to turn in a little early to get down to the apex, and emphasize a later apex. Work up to it. This is a fast corner and it leads on to a decent straight so the exit and getting back to power is more important than carrying a lot of entry speed. If you find yourself at a loss and you know you are going off the track; drive off straight and bring the car back on the track down the road. Pinching the exit and dropping a few wheels can lead to a hook-spin. Fixing or replacing a splitter is a lot cheaper than bent wheels, or a banged up roof. Buttonwillow is known for its powdery roll-inducing dirt, try to prevent going off sideways at all costs.
Turn 15 (Sweeper)
In my opinion this is one of the harder corners on the track to do it fast. The sweeper is a flat, long, sweeping corner that really brings out any imbalance of the car’s setup. The key to do this corner fast is entryspeed, entryspeed, entryspeed. Too little and conservative entry speed leaves time on the table. The problem with this corner is, too much entryspeed will result in you sailing off the track. Carrying a lot of speed into a corner while trying to make the corner and get the car slowed down isn’t always the easiest. A proper line will help the entry and make the corner less of a knife-edge between slow and going off the track. But when done right, it’s still pretty close.
Brake briefly in a straight line on the left side of the road and turn-in trailing the brakes to an early apex. Hug the inside of the road until roughly half way through the corner. By this point you may go back to maintenance throttle to keep the mid-corner speeds up while tracking out to the left side (outside) of the corner. Hug the outer edge of the road until you can see the apex. Once you can see the apex, be patient and turn-in later for a late apex as the sweeper (like Riverside) is also a decreasing-radius corner. As you approach the apex, squeeze the power to full throttle. Track out up and over the entire curbing on the left into the esses.
Note: I left a ton of time on the table at the entry of the sweeper in my NSX in-car footage.