How do i lock the rpm/speed of a car?

I watched a YBR video about the 0.10 update, he came to a part where he puts to drag cars next to each other. Then he pulled up the radical menu and enabled the transbrake. Then he put the RPM to Max and I am wondering how he did it?

 

Watch the video again

And see how he did it

 

Not exactly sure what you're asking, but you will need to manually assign the transbrake a key or button in the controls menu otherwise you have to go through the radial menu each time you want to use it.

 

--- Post updated ---
how do you control 2 cars at the same time

 

You cant control both cars at the same time without multiseat, but what he did is set the throttle, and then before releasing it switched cars. This allows him to effectively have a drag race.