If breaking up through resistance is a powerful buy signal, then breaking down through support is a powerful sell signal. Remembering the trampoline analogy, once the floor gives way, there is nothing to hold you there. Gravity, or in the case of the share market, negative market sentiment takes hold and you are in freefall to the next level.

A good example of this is shown below. This company had been in a declining trend.

The first trading level was at around $7.80. Once this support level was broken, the price quickly fell to a level of around $6.00. Buyers supported this level for a period, but again this support was broken and the price fell further. Support was broken 5 times in less than a year, during which time the share price halved. The last support level on this chart was around $4.00. The company rose in value off this price but fell back to it not long after. However, up to the date of the production of this help file, this is still the support line for this stock as it has not been broken. Investors who own the stock are not prepared to sell at a lower price than this and buyers see this as a value buy at this level.
As a trader, you want to avoid losses of 50%. To do this you must sell. The SELL signal comes when the price closes below the series of recent troughs that make a support line. If you ignore the first, then your loss would have been over 25%. Ignoring the 2nd, 3rd, 4th and 5th sell signals increases this loss to 35%, 40%, 43% and 47% respectively.