You're probably referring to the NO2 products out there. They do indeed cause blood to flow into the muscles, giving you a short lived 'pump'. that's all those NO2 products do. there is no other benefit to actual training.
but they keep selling because its one of the few 'supplements' that you can actually feel/see, and it works in about 30 minutes.
but it doesn't DO anything useful, so its just a waste of money, unless you're into vanity.