What does the throttle valve lever do?

The TV lever basically does three things:

1: It makes the transmission line pressure increase when you increase the throttle. When you step into the throttle for more power the line pressure goes up so the clutches and bands don't slip. 
 
2: It will delay the shifts with increased throttle. You wouldn’t want the transmission to be in high gear at 40 MPH at wide open throttle, but you would want the transmission to be in high gear at light throttle.
 
3: At light throttle, the TV lever, along with governor pressure and the 1-2 shift control valve, dictate how fast the front band is released. The band is released quickly at light throttle so there is no bind up. The front band is released slower at wide open throttle so there is no cut loose or engine flare.
 
With a ranging-pressure manual valve body, you would have the TV lever adjusted just like a normal valve body for smooth shifts at light throttle, and quicker shifts at full throttle. 
 
With a constant-pressure manual valve body, the only thing the TV lever will do is release the front band sooner at light throttle, or slower at heavy throttle. We would use an old choke cable, such as an old carburetor, and connect it to the TV lever. At light throttle, the lever would be all the way forward for a smoother 2-3 shift. At full throttle, you would pull the choke cable so the lever is all the way back so there was not a flare during the shift.