A hammer that occurs after an up trend is called a 'hanging man' and is a bearish signal.
The Hanging Man candlestick formation, as one could predict from the name, is a bearish sign. This pattern occurs mainly at the top of up trends and is a warning of a potential reversal downward.
It is important to emphasize that the Hanging Man pattern is a warning of potential price change, not a signal, in and of itself, to go short.
The Hanging Man formation, just like the Hammer, is created when the open, high, and close are roughly the same price. Also, there is a long lower shadow, which should be at least twice the length of the real body.
When the high and the open are the same, a bearish Hanging Man candlestick is formed and it is considered a stronger bearish sign than when the high and close are the same, forming a bullish Hanging Man (the bullish Hanging Man is still bearish, just less so because the day closed with gains).
After a long uptrend, the formation of a Hanging Man is bearish because prices hesitated by dropping significantly during the day. Granted, buyers came back into the stock, future, or currency and pushed price back near the open, but the fact that prices were able to fall significantly shows that bears are testing the resolve of the bulls. What happens on the next day after the Hanging Man pattern is what gives traders an idea as to whether or not prices will go higher or lower.
The chart below of Alcoa (AA) stock illustrates a Hanging Man, and the large red bearish candle after the Hanging Man strengthens the bears thinking that a downward reversal is coming:
In the chart above of Alcoa, the market began the day testing to find where demand would enter the market. Alcoa's stock price eventually found support at the low of the day. The bears' excursion downward was halted and prices ended the day slightly above the close.
Confirmation that the uptrend was in trouble occurred when Alcoa gapped down the next day and continued downward creating a large bearish red candle. To some traders, this confirmation candle, plus the fact that the upward trend line support was broken, gave the signal to go short.
It is important to repeat, that the Hanging Man formation is not the sign to go short; other indicators such as a trend line break or confirmation candle should be used to generate sell signals.