I tried doing some research on this a few years ago and actually did it in a race.
Some posts are correct in that he gives you the poops.
Basically went I ended up doing was taking about a couple spoon fulls mixed with a decent amount of water every 30 minutes starting at at about 3 hours before race. I ingested quite a bit of baking soda but was able to crap my brains out a couple of times prior to the race. Felt alright during the race.
At the time I was 23 with a PR of 2:03 in the 800m from senior year of high school but ran 2:01 that day. I did not run in college at all and at the time my training basically consisted of of some interval work like 2 or 3 days a week and thats it. I had no races prior to that one so I have no real idea if it helped at all and haven't tried it since then but might be something to it I dont know.