[Hot Wheels]

I think what you are seeing is that at 4200 rpm or so you are losing traction and at that point the engine is revving very quickly to either the shift point in drive or the rev limiter in autostick and then shifts into second. Normally, with good traction, you might shift at 40-45 mph without any bog. When the tires spin, you are only going about 25 mph and the bog occurs because there is too much of a difference between the speed that the car is actually going and what the car's gearing is expecting. If you have any mods that cause you to lose low-end torque, such as a very low back-pressure exhaust system, the bog will be even worse.