I'm not sure about that.
My coworkers are arguing that a bunch of people are buying put options, effectively shorting the stock market, in the days prior to these FOMC meetings. At 2pm, the meeting notes come out, and we see that the expected .75% rate happened.
Since that was "expected", all the put options are now sold. That causes the stock market to jump up (since the effective-short positions are liquidated).
Its just a hedge, just in case the numbers come in and the Fed chooses like 1% hike or higher instead of the expected .75%.