Hi, long time no see. The ideal of donating blood means all blood goes. None is differed.
Cupping or Hijama is pumping. So if you want to fill a air mattress with air you might use a foot pump. So you pump and pump to put air in the mattress.
The same principal with hijama, a cup is placed lets say on the inside of the wrist where all your blood vessels are. This is the area where many commit suicide by using a rasor blade to cut thru all the veins. Now the specialist, places a dome shaped cup and it has a pump on it. like a water pump and she pumps and pumps until it almost hurts and raises up an area lets say of a tea cup. The bad blood come to the top and then when she cuts you the bad blood being on top is expelled. I know maybe this sounds strange, but I have a link which should explain a little more and it has many links and a video to watch.
Blood letting was done on crazy people years ago and supposedly helped their situations. My mom goes bonkers everytime I do this as she thinks I am going to die. Make sure the person doing this is professional using clean instruments as you know that bacteria can kill.