What happens if you temporally cut off blood flow to your head?

like i mean for 2 mins. my sister did this with some ace bandage thingy to see how red her face would get or something and she like fell down and was unresponsive for like a 2 mins after i yanked the ace bandage off of her. she said she doesn't remember like anything at all . but what actually happens to make you fall and now she has this lump behind her head.. she dumb. so u don't get the wrong idea she is older than me i didn't advise her to do thiss lol