what is a low processing power transformation function?

sha256 is an example of a low processing transformation function.

it is told that a low processing transformation function can transform information in so much, that it requires impossible amount of energy to find the original information after the tiny amount of energy used for the transformation.

from understanding the facts behind generation of information I=P, where amount of new information is always directly related to the amount of computing power used, the statement in previous paragraph is absurd.