Hi, this is kapalbhati (I attached the link with the explanation), it is not dangerous and great to practice at the office during the day to clear the head and calm the mind. There is also the alternate nostril breathing technique. I used to practice both before stressfull meetings and then things would kind of solve themselves, cause I was more focused, centered.http://earthyogi.blogspot.com/2011/06/kapalbhati-everything-about-this.html
The alternate breathing is not dangerous if there is no retention involved. Breath out for four counts out of the left nostril, then breathe in for four counts from the left as well, breathe out on the right for four counts and breathe in for four counts on the right. Continue for 6 rounds or 12. If you feel discomfort stop.
If one of your nostrils is blocked you may need to unblock it though... good techniques:
Lay on the opposite side of the blocked nostril for a while, or do some kapalbhati as explained in that link
Good luck. Would love to hear if it works!