For me it was because I was short EBAY, INTU, INFS and TRDO from previous day(s) and with the exception of EBAY the gap up was not enough to touch my stop. I then added to the good ones when the gap failed.
I also went in with CPQ, USON PKS and GLBL long. I dumped half my CPQ and all my USON as they opened weak, considering the gap. The rest of CPQ went when my stop was hit, but I kept the other two all day.
I am a swing trader, so I didn't need to make lightening fast decisions to take advantage. The decisions had already been made the day(s) before and the market cooperated.
However, I gave a quarter of it back today as all my shorts and GLBL were stopped out and a couple of other ventures didn't pan out. Good gains in INFS and PKS were not enough to offset the damage. I guess the market didn't cooperate.