Good questions. First off, I didn't know it was going to go down. This is a very important point. However, a descending triangle - especially following a fall - is usually followed by a further fall (basically continuation of the larger market direction). The correlation is even higher when it breaks down through the base of the triangle. I do not know the actual statistics to back this up, but triangular consolidations are very strong indicators. As to the target price, that is simply calculated by measuring the height of the triangle (in this case, about 50 points), and subtracting from the base level of the triangle (in this case, ~9860).
There's no great shakes to this system, but, once you can recognize the patterns, it does require the discipline (a) to wait for a stong consolidation pattern to form, and (b) to wait for the break-out from the pattern rather than try and anticipate it (very tempting to try and make a few more points, but you can get caught out when the formation does not complete as you anticipated). In addition, as I mentioned earlier, you need to get a bit of prior experience as to what happens shortly after the break-out to trade it properly and confidently. This is not something that can be easily taught, rather it is something that needs to be learnt by observation.
Hope that is of some help.
Alex