Lost is coming on, so I don't have time to explain a stop and go, but in this case, you don't know your opponent has AK until after the fact. Most likely with JJ, I'm getting all the money in pre-flop if I'm short. If he calls and flips AK, then you're racing. If he hits an A or K, it was close enough to 50/50 - not a big deal.
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