In the RotK movie, Gandalf and Merry light up the beacons of Gondor. These beacons have been built atop each peak of the White Mountains. Mysteriously, when one beacon is lit another also becomes lit as if by magic.
What's the underlying technology here? How do these beacons work?
No magic. In the relevant scene, you can see two of the beacons close up being lit:
There's a bowl of oil and a lamp above the first one. Pour the oil onto the wood pile, and light it with the lamp.
A person with a lit torch holds the torch to the second one.
The technology is "fire". Each beacon is staffed by watchers (two in each of the cases we see), whose responsibility includes looking for beacons along the chain being lit and lighting theirs in response.
