The prints that are going off are mostly dark books. "ecn's" that are mostly institutional in origin. Some prop firms have access to these and can peg the bid or ask. So, you get filled at 3.485 and you immediately offer out at 3.485 or something similar. But, because you entered and exited the trade passively, you get ECN rebates. Sometimes when pegging the bid/ask you'll do better and can actually capture some of the spread. A buddy of mine made 18k the other day doing this exact thing and never even made a full cent - albeit, he did several million shares that day but, never once had to punch out.