RF is actually quite simple. When a soldier has active RF during alien turn and an alien spends TU in his field of fire equal to TU cost of the selected RF mode, soldier will fire. Same goes for aliens during your turn. The only exception I have seen so far are hovernets, whose fire apparently won't trigger RF, but their movement will.
This has few implications. AFAIK no weapon except pistols have fire mode with TU cost less than 8, making grenades very useful. Preparing flashbang, going in and throwing flashbang in generally safe, and flashbang exploding close to alien has high chance of removing his TU for RF. I don't know exact formula, but for some odd reason it seems they work best when they explode at cell next to alien, sometimes they seem to fail when they explode right on his cell, or too far. Grenades are also good option when you need to kill alien quick, but watch for splash damage.
Also, moving out to the side of soldier/alien before firing works too if you can do it quick enough. Aliens will try to do this frequently on close range, like when fighting inside buildings or UFOs. This can be avoided by proper positioning and multiple soldiers covering each other. Generally, watch the TU you're spending in enemy field of fire. Under 8 it's okay, if you think you'd go over 8 try to use flashbang or cover your movement with smoke grenade instead.
