The things that can stop the boost timer.

Transbrake Activation - only activates a temporary boost value.
If you use dome pressure, it is in dome pressure values, if you use Dome+Map it will be in actual boost values.
The Timer will Reset to 0 when you activate this switch and start the timer when you release the switch.

TPS To Start Modulation Ramp - anytime you are under this value, your boost timer stops (but does not reset).
Conversely, it is also a value that will adjust your boost value based on pedal position by a percentage.
Example: Set TPS modulation to 50%. At 50% TPS you will get 0% of your commanded boost value,
at 75% TPS you will get 50% of your boost value, and at 100% TPS you will get 100% of your boost value.
To make it more clear, command boost of 10 psi.
At 50% TPS, you get 0 psi boost, at 75% TPS you would get 5 psi boost, at 100% TPS you would get 10 psi boost.
If you were to set the value to TPS of 75% to start the boost modulation, you would get 0 psi at 75% TPS, and Ramp to 10 psi at 100% TPS.

Otherwise, ANY TIME your Boost enable Switch is active, the Boost Timer Starts in Boost vs Time & Boost Manual.
the only difference with boost manual, is that it's designed for stick cars (hence the "Manual" transmission reference) which typically can not
handle all of the boost in lower gears, so they often choose a lower boost in 1st gear, and on the rpm drop (showing the ECU a gear shift)
it would move to stage 2 (or 2nd gear) and then to stage 3 (or 4rd gear), etc, etc.