QRFs (Quick Reaction Forces) are a fancy name for a "battle". When capturing a base or town/city one will be triggered and a notification will be sent to all players.
Only one QRF runs anywhere on the map at a time. While one is active, the action that starts another is not offered.
1. Reinforcements are sent. The occupying faction allocates soldiers and hardware out of its reserves and sends them in from every other base it still holds. How big the force is depends on how much it has in reserve, capped by a difficulty setting and scaled up by the number of players online. There is a minimum size, so even a broke occupying faction sends something.
2. A countdown runs while those troops arrive and take up positions. Nothing is scored yet.
3. Then the ground is counted. Every few seconds the game compares how many players are inside the contested area with how many occupying soldiers are there with them:
4. Somebody reaches the total. The count runs to a number the difficulty settings set, in either direction, and whoever gets there wins. Only then does the location actually change hands, and only if the winner is not already holding it.
Players inside the fighting earn experience for as long as it goes on. You can track the progress of the QRF at the top of your screen when one is active.
This depends on a difficulty setting, and servers differ:
You may still respawn inside that zone if your home is there.
Tip: If you know a QRF is coming, set your home to somewhere nearby or it may become very difficult to get back into the fight when you die
QRFs are not only something you trigger. When the occupying faction has more in reserve than it needs, it will pick a base the resistance has taken and launch its own QRF to get it back, then hold off for a while before trying again. Towns are fought over the same way.