It takes minimal energy to deorbit from LEO. By changing the shape of the orbit you spend some time in denser atmosphere which increases drag which quickly deorbits you. Controlled just means they start the processes at a specific time.

How much reserve fuel does an out of service satellite need to use after it burns up in Earth's atmosphere?

How much fuel do you imagine it takes a satellite to re-enter if it will happen automatically in a matter of a few years?

The answer to both is, of course, not much, not much at all.