I used to do this all the time as did my daughter. When I asked the pediatrician about it out of curiosity, they explained it really well. Imagine the veins/arteries in your neck as straws you drink soda with. Now i.agine during one of these crying spells they go from that to the size of a coffee stir straw. Those veins become comstricted and essentially cut blow flow to the brain. As soon as my daughter would pass out, everything relaxed and blood flow returned to normal.