Reflected-wave switching from Wikipedia contains the following paragraph:
If, on the other hand, there is no termination at the end of the microstrip, and the pulse encounters an open circuit, it is reflected back towards its source. As this reflected wave travels back along the microstrip, its amplitude is added to that of the original pulse. As the reflected wave passes the receiver for a second time, this time from the opposite direction, it now has enough amplitude to be detected. This is what happens in a reflected-wave switching bus.
The bus has to be short enough, such that a pulse may travel twice the length of the backplane (one complete journey for the incident wave, and another for the reflected wave), and stabilize sufficiently to be read in a single bus cycle. The travel time can be calculated by dividing the round-trip length of the bus by the speed of propagation of the signal (which is roughly one half to two-thirds of c, the speed of light in vacuum).
Below is another place talking about the same thing (within the book PCI Express Technology Comprehensive Guide to Generations 1.x, 2.x, 3.0, by Mike Jackson and Ravi Budruk):
I don't quite understand the reason behind the "round-trip" calculation: It seems that for the receiver to detect the signal, it is only necessary that the signal level is doubled by the reflection. Why does the receiver need to wait until the reflection travels back to the source and is absorbed?