Why in some Siddurim after the marital duties, it is written that we should throw a little bit of water on the floor, next to the bed?

In continuation with related material to Sod HaZivug, Kabbalah brings that, “When one has eaten and is satisfied in a seudah, it is necessary to give the scraps and the dregs to the sitra achra, and any particles left upon the hands after a meal must be washed away [with the water called mayin achronim] so that the ‘other side’ may receive its due. Therefore the washing of the hands after a meal is an imperative duty [in order to avoid the ‘tribulations’ of the sitra achra. See Kaf HaChayim Sofer 181:8] and the water must be poured away into an appropriate place” (Zohar II:154b, Terumah; Talmud, Chulin 105a-b).

Now, the negative spiritual element that seeks to ruin man through constantly seducing him sexually to procreate (from the zera levatalah, as already explained) its progeny of shedim, and “who lives in the depth of the great abyss of the ocean… is a certain hot fiery female spirit named Lilit y’s“ (Zohar III:19a, Vayikra). If man hears the Torah’s pronouncement, Kedoshim tihiyu, Be holy!, and treat the sexual act with his wife with awe of G-d, this most destructive of spirits does not have authorization to come close and injure, chaz v’shalom, this man and his loved ones - for it is hindered by three malachim. However, if the man is in fact seduced to stray off the praiseworthy path of the L-ord, “then, as a fool, he comes close to her [Lilit y’s]. She dresses with jewelry like an abominable prostitute, hugs and kisses him, and serves him wine filled with dregs and the remains of the Serpent’s venom. And when she sees him desiring her and moving away from the path of truth, she removes all of her adornments she worn for him… She then leaves him sleeping and goes up High in order to denounce and accuse him, so as to receive permission to destroy him. The fool then awakens, thinking he will have her again with all of his pleasurable fancies, but he sees her without her sweet vestments, for now she has become a powerful oppressor dressed in a hot and fiery garment, causing this fool great terror to the whole of his body and soul. This oppressor has terrible eyes, and a sharp sword with drops of venom dripping from it… She kills the fool and throws him in Gehinom” (Zohar I:148a-b, Sitrei Torah).

As such, it is vital to know that Lilit y”s is always on the look out wishing to damage the world, seducing the ego of men with the illusion of our material existence. In fact, even when man unites with his wife in holiness to procreate, it is necessary to know the “medicine” to ward off this shed from any opportunity to bring harm, chaz v’shalom. In a low tone and “with the heart devout to Hashem, should the mane pronounce [in Aramaic]: Atifa be’kitefa izdamnat, sha’arei sha’arei, lah tiol ve’lah tinpok, la didan ve’lah be’advan. Tuv tuv, iamah it’reguisha, galgaloi lin karan, be’chulakah kadisha achidnah bikdusha de’malka it’atafna. ‘He who is covered by a sheet [to wit, Lilit y’s, who is always covered and wailing, for the name Lilit y’s is derived from crying, in Hebrew, yelalah] is here. Go far away from me, go far away from me. Ye shall not enter nor leave. This is not yours, nor does it belong to your portion. Return, return, the waters revolt, its waves expect you back. I connect to the holy portion. I am covered with the holiness of the King’. The man should cover his and his wife’s head for a short period, and then they should unite. After the act, it is necessary to spill a bit of water around the bed. This is the best protection” (Zohar III:19a, Vayikra), since it is as much her barrier and her portion! It is her mayin achronim. In this way, man ensures that she will only take the drops of water, but not the “drops of life” (i.e., the vital and creative force of semen).