Well, the answer is not inside accepted science.
When you lift weights, you not only become stronger, the weights become lighter.
It is a belief thing.
Not only can your muscles work far more efficiently, but when something needs 'doing' and your belief in that 'doing' is stronger than your belief in gravity, then you can lift that car off your child.
And yes, it can be trained.
When I think of the words believe and belief, I tend to associate them with prior contemplation and/or experience. With that in mind, I imagine that a person wouldn’t really be stopping to think about their beliefs and assumptions when faced with an emergency like having to lift a car off a child. Because of that, it’s hard for me to grasp the way you’ve explained the above comment.
I open to it, though. How can what you are speaking of be trained?