It transmits a signal and then looks at what is recieved .How this is looked at and analysed in done by an inbuilt computer.
Minelab, an Australian company , took the guesswork and complexity out of metal detecting.
It raised the benchmark quite a few pegs and the American competition is still way behind.
It is the software in the computer that is the secret to Minelabs success , not how good its vlf Rx is.