How does Hijama help against Sihr?
The harm caused by Jinn possession ends when you are treated by making Ruqia to yourself or by a Muslim scholar specialized in removing Jinn via Ruqya and when the substance of Sihr inside the body is removed via a specialized Hijama 
Practitioner, vomiting or emptying the bowels in the bathroom.
Some times Jinn leaves the body right before Hijama is performed or even during Hijama if the Hijama Practitioner places the cups on the areas of the body where the Jinn knots are. If the Possession with Jinn did not resolve before Hijama, Hijama could end up hurting the jinn itself. This is why sometimes people who have active jinn obsession experience shaking, screaming or faint during Hijama. It is all from the effect of Jinn as it tries to scare the patient away from doing Hijama or Ruqya. When the cups are placed on the areas where the jinn is, the Jinn could make this person feel pain after Hijama as a way of keeping this person scared away from repeating Hijama.
In an authentic statement (Hadith) by prophet Muhammed, “The devil runs inside the son of Adam, wherever his blood flows.” This is why after Jinn possession ends, it leaves behind harmful substances inside the body that is felt in the form of aches in the nerves, muscles, blood vessels etc. If the Hijama cup is placed on the right spots where the jinn left these harmful substances, it will come out with the blood of Hijama. Hijama removes various types of black magic as in the case of eaten, drunk, smelt or sprayed Sihr. Once Sihr enters the body, it spreads through the blood to various body organs, especially the areas sihr was intended for. When the treatment is complete, the victim of Sihr will typically start vomiting or experience a diarrhea as means of clearing the body of any remainder of the black magic.
As for the effect of Hijama on Iyen (Evil Eye), Iyen tends to be more localized in the body. Hijama can alleviate the damage of the Evil Eye unless it has caused permanent damage beforehand.