"Multiplier" implies an output greater than the sum of the inputs, which is impossible. Perhaps it is just a misnomer.
Nope, not a misnomer. Stellar radiation, upon striking aluminium, radiates 2-3x more than input. Nasa had an engine designed, for satellites and interplanetary observatories, that used a radiation multiplier propulsion engine (which produced 8x output), through stages I believe.
2-3x the input you say? So we can just take a small piece of aluminium heat it up with a lighter, use that piece of aluminium to heat up 3 new pieces, use these three pieces to heat up 9 pieces of aluminum etc
Then we could have a room full of very hot aluminum and use it to boil water. The steam could then be used to generate electricity. Awesome a simple lighter and some aluminium could replace a nuclear power plant.
But there are more ways to get free energy, just check out youtube.