The definition
An engulfing pattern is two candles of opposite colour where the second candle's body completely covers the first candle's body — its open is beyond the prior close and its close is beyond the prior open.
Note the emphasis on bodies. The pattern is defined by the bodies, not the wicks. Many patterns that look engulfing on a glance are not, because one wick pokes out.
Why it is the most useful of the two-candle patterns
Because it is a genuine change of character rather than just a shape. A bearish candle followed by a larger bullish candle that opens below the prior close and closes above the prior open means: sellers controlled the last period completely, then buyers took over so thoroughly that they erased the entire prior range and more. That is a real transfer of control, not an inference from a wick.
Factors that make it stronger
- Size contrast. A tiny first candle engulfed by a very large second one is a much stronger statement than two similar-sized candles.
- The prior trend. A bullish engulfing after a clear decline is meaningful; the same pattern mid-range usually is not.
- Volume or range expansion. If your platform gives usable volume, an engulfing candle on above-average volume carries more weight. Be aware that on many retail CFD platforms the volume feed is the broker's tick volume, indicative rather than exact.
- Where it lands. At a known support or resistance level, an engulfing pattern is far more informative than in open space.
The honest caveat
Engulfing patterns are common. On a 5-minute chart they appear several times a day, and the overwhelming majority lead nowhere. Their value comes from rarity, which means higher timeframes and levels. An engulfing pattern on the daily at a level you already marked is worth your attention. The tenth one on the 5-minute chart today is not.
How to check this yourself
Go to a daily chart and mark every engulfing pattern in the last year — there will be far fewer than you expect, which is the point. For each, note whether it appeared at a level and what happened over the next five bars. Then do the same on a 15-minute chart for the last week and compare how many there were. The ratio tells you more about timeframe selection than any rule I could give you.