You can use it for different things. my main use for it is to project targets based on impuls legs. not always will an ABCD pattern have the AB=CD 1:1 ratio. CD will often be in a fib ratio to AB like indicated on the chart. using the tool you can find different levels quickly and even set your own custom level values.
You can find it in your drawing toolbar by clicking the third icon and selecting "Trend-Based Fib Extension"