I dont understand what you are asking. If you are talking about a typical stock hlats on NYSE, it happens because of some significant news. It can gap up or down depending on the news, of course. When a news hits the market ( a bad news, let's say), there will be tons of sell orders, naturally. If the specialist can not handle the order orderly, he will halt the stock (order imbalance). A stock that is halted based on order imbalance can continue to trade on ECN's like SILD and ARCA....but if it halted because of NEWS PENDING, then the stock halts trading everywhere.
Anyway, once the stock gets halted, the specialist will gather all the open orders, sets a "fair" price - somewhere below last trade - and then matches the orders and re-opens it at some price. Depending on what the news is, the stock will open up or down. Sometimes, its tricky...like GM last month. It started to tank based on accounting rumors....and then was halted with the new quote indicated $2 below last trade... When it re-opened, it opened UP $1 or whatever because the rumor was disputed by GM during the halt... or something to that effect. Suffice it to say, it opened higher than the last trade before the halt ( last was $51.50). I bought at at $52.30 and $51.50 during the downtrend... and when it got halted and showed $49 - 51, I dumped all my shares at $50 on ECN AFTER the hlat on NYSE. The stock opened UP at $52.50..and then second print was $53.50. It was NOT a good day.