Namuna & Nox,
The difference in the boost .vs non-boost circuits is based on the TX2C chip. What happens is that the TX chip puts out a series of pulses in a pre-determined sequence depending on what button you push. This is known an OOK (On-Off Keying). The turbo button is just another OOK pattern that is transmitted to the car.
On the RX circuit the RX2C chip detects what OOK patten has been received and drives the output to suit. In this case the RX board prolly has a different H-Bridge design (H-bridges are use to supply current to a motor in one of two directions, hence fwd & rev) that accounts for the boost funtion.
More info can be obtained from the tx2/rx2 data sheet
here .
More info can be found generally
here .
Cheers,
ph2t.