It's worth googling and keeping in mind.
I think generally the gap needs to happen between two consecutive trading days, and you just don't want (for example) a high from yesterday being less than today's low. cause that makes a price gap where it didn't just run one pip at a time, and invariably, the gaps get filled
sure, it doesn't always play out but you'd be surprised how many times it does.
LOL, I may be waiting forever for a number of gaps to fill lower with NCM, it's just taken right off and does not seem to give a fig.