A junkie term used to describe when you slowly pull on the plunger on the syringe, the blood will shoot into the syringe, having a flag waving appearance. This is a way of knowing that you've hit a vein.
remember kiddies, if it's red(dark red, vein) go ahead, if it's pink(bright red, artery) stop and think
Sarah knew she hit a vein when she saw the flag. She then pushed on the plunger to get the H in her system.