Is this activity dangerous or unhealthy?

My friend showed me this activity where he would take fast short breaths until he felt woozy and then he would push on his neck with his hands and tighten his abs increasing blood flow to the brain making him pass out for a couple of seconds. Is this very dangerous or unhealthy?

Note: I understand that doing similar activities to this one with any type of cord or belt or strap wrapped around the neck is extremely dangerous so lets not talk about that.