When a market makes a move in any direction it tends to slow down or lose momentum at some point prior to moving in the opposite direction for a period of time, before the previous trend is resumed. These counter trend moves are known as retracements and may occur when a market is moving either up or down.
The reason they occur at all is that traders are never in unanimous agreement about the direction of the market and the opinions that matter; orders entered into the market reflect that disagreement. The upper hand is gained by those trading with the trend but that is almost always subject to temporary reversals.
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