People who are afraid of ayin hara are worried that if someone is too beautiful, successful, etc., others will be jealous and it will be as if in Shamayim they will “re-think” if you really deserve that bracha. So, if you show that they are not something so great, there will be no need to re-judge them in Shamayim. Poo Poo is like spitting on the person to show they are not so great.
Something along those lines.