Energy is trapped by producers.(from the sun)
This energy cannot be reverted back to the sun.
It steadily goes to the next tropic level. And thus energy flows constantly in only one direction.Thus,unidirectional.
Also,biological magnification is how pesticides enter human body.
That is, harmful chemical like pesticides get mixed with water or soil.Since they are non biodegradable;they get concentrated and accumulated in every trophic level.The tertiary consumers being humans get the most amount of pesticides.