The BWT time period is the time that elapses when a dark pixel is activated, lights up brightly and falls back into the dark state. In other words, it goes through a black-white-black cycle. It is the sum of the rise time that the pixel takes to change from the dark to the light state, and the fall time when the drive voltage is switched off and the state changes from light to dark. These two times are not identical, because the pixel is still luminous when it is switched off. Which is why the fall time is longer than the rise time.
If the values for the reaction time are given, then the rise time is given first and then the fall time.