This sets how long a backward pulse signal stays on before being turned off, applying to incoming R2 registers where backward signals are sent as pulses, in semi-compelled or non-compelled mode, and also in compelled signaling for the specific backward signals that are pulsed rather than held. The breadth of signaling modes this single timer covers, three different R2 modes, means it is a shared, general-purpose pulse-width setting rather than mode-specific, so changing it affects backward pulse timing across whichever of those modes the trunk happens to use. On a trunk running semi-compelled R2 signaling, this timer set to 100 ms would define the width of every backward pulse the gateway sends in that mode, regardless of which specific backward signal is being pulsed.