There are four types of gaps, excluding the gap that occurs as a result of a stock going ex-dividend. Each type has its own distinctive implication so it is important to be able to distinguish between them.
It is quite possible that confusion between measuring gap and exhaustion gap can cause an investor to position himself incorrectly and to miss significant gains during the last half of a major uptrend. Keeping an eye on the volume can help to find the clue between measuring gap and exhaustion gap. Normally, noticeable heavy volume accompanies the arrival of exhaustion gap.
Some market speculators "Fade" the gap on the opening of a market. This means for example that if the S&P 500 closed the day before at 1150 (16:15 EST) and opens today at 1160 (09:30 EST), they will short the market expecting this "upgap" to close. A "downgap" would mean today opens at, for example, 1140, and the speculator buys the market at the open expecting the "downgap to close". The probability of this happening on any given day is depending on the market. Once the probability of "gap fill" on any given day or technical position is established, then the best setups for this trade can be identified. Some days have such a low probability of the gap filling that speculators will trade in the direction of the gap.