The saahir (person who practises sihr) does things by which he affects people with sihr. Sometimes this is by illusion or making things appear to be something else, as Allaah said concerning the sorcerers of Pharaoh (interpretation of the meaning):
“and their sticks, by their magic, appeared to him as though they moved fast” [Ta-Ha 20:66]
They do actions that change how things look in people’s eyes, so that they see things other than they really are, as Allaah says in Soorat al-A’raaf (interpretation of the meaning):
“So when they threw, they bewitched the eyes of the people, and struck terror into them, and they displayed a great magic” [al-A’raaf 7:116]
So they do things to bewitch people’s eyes, so that they see a rope as a snake or a stick as a moving snake, when it is not a snake at all but only a stick or a rope.
They may also bewitch people in other ways, so that a man becomes hateful to his wife, or vice versa. They may do this by bewitching their eyes, or by giving them evil potions which they get from the shayaateen, or by tying knots on which they blow and pray to the shayaateen instead of to Allaah, and seek their help in harming people. So a man may imagine that his wife is not the wife he knows, and she may appear ugly or hateful to him, or a wife may imagine that her husband is not the husband that she knows, and she sees him as something ugly and terrifying, because of what these criminals have done. So their sihr may be of two types, one of which involves illusions and tricking the eyes, so that they do not see things as they really are.
The other kind includes that which is called al-sarf and al-‘atf (spells aimed at creating hatred or love), which is done by tying knots and blowing on them, and using potions which they make by inspiration of the shayaateen. This second kind may make a man love or hate his wife, or a woman love or hate her husband. It may also affect a person's relationship with people other than his or her spouse. Hence Allaah has commanded us to seek refuge with Him from the evil of those who blow onto knots, and to seek refuge with Him from every evil.
Majmoo’ Fataawa wa Maqaalaat Mutanawwi’ah li Samaahat al-Shaykh al-‘Allaamah ‘Abd al-‘Azeez ibn ‘Abd-Allaah ibn Baaz (may Allaah have mercy on him), vol. 8, p. 80