Tuesday was a misty day when I got in with my student again. He had certainly improved overnight (this is typical). Things were going well and I was getting ready to sign him off to get a check ride to run solo. I wanted to work on a couple more things before doing that, so I addressed them in that session. He had been braking very early at the end of the back straight, so I gave him a reference point, which was still early, but about 1 ½ car-lengths later than where had been braking. It was a jut in the wall, which I told him to consider as the 5 marker, after which there were cones indicating 4, 3, 2, and 1. The next lap around, he braked precisely at the 5 marker and had plenty of room to slow. I was pleased with that. However, on the next lap, I did not say anything more, but as we approached the braking zone at 120 mph, he passed his original braking point, then passed the 5 marker, at which point my eyes went wide, and I was yelling BRAKES!!! as he was passing the 4 marker. The 4 marker is pretty much the absolute limit for a car of his speed. He buried both feet into the brake pedal and got the car slowed down by the outside of the turn. I looked at him and asked what had happened. Since he had been able to slow in time on the previous lap, he had taken it upon himself to push his braking zone back by the same amount again. We took a slow lap or two while I explained the exponential nature of pushing the limits. At first you can make large gains, but then you must push the limits in smaller and smaller increments so that you can still correct yourself when you just step over them. I reminded him to not try anything new like that without first telling his instructor and definitely not during his check-ride. Consistency is key and keeping their instructor calm should be a primary goal of a student. I did sign him off to run solo and his check-ride instructor approved, as well. He completed the day without issue and had a car in one piece to drive back to Minnesota with.