Chasidut su Salmi 51:7

Or one may say - at evening [ohr] of the 14th we check for chametz by the light [ohr] of a candle. It is explained that the essence of chametz is the yetzer hara, as explained above, and as explained in the gemarah "who prevents? The yeast in the dough". And behold the essence of the chametz is the yetzer hara, which comes because the building-block of humans is desire and pleasure of sex, as David Hamelech peace be upon him, says: " Indeed I was born with iniquity; with sin my mother conceived me" (Psalms 51:7), therefore a person needs to sanctify oneself at the moment of sex since the more one sanctifies oneself the more one gives to the fetus a holy soul, since from the holiness of sex a yetzer tov comes to the fetus. And from desire and lust comes the yetzer hara, therefore one should diminish one's desire so as to diminish the yetzer hara of the fetus, and the yetzer hatov can conquer with ease the yetzer hara. And regarding this it is written "at evening [ohr] of the 14th we search for chametz by the light [ohr] of a candle - that is to say, by the light of one's soul, since "the candle of Hashem is the soul of a person", since candle [ner] is the acronym of "neshamah, ruach" and "nefesh, ruach", which is appropriate for searching and burning the chametz completely.