What exactly is 'empowerment' and 'lung'?

Sometimes, gurus will perform empowerment ceremonies or give lung in-person (or more recently, online) to recite a certain mantra or practice a certain deity. Sometimes this comes with samaya vows. What exactly is empowerment & lung? It is said that reciting certain mantra without empowerment is ineffective, possibly even useless. It seems that empowerment 'activates' certain aspects in the practitioner that makes the mantra 'useful'. I've heard the analogy is driving a car without fuel & driving one with fuel to describe the difference between mantra without & with empowerment. What is it about the empowerment that makes the mantra useful? What is the mechanics of this, how does this happen?