In a Trend
An expanding triangle (XT) signals the end of a bull trend by taking out a higher low (HL) and vice versa on a bear.
Expanding triangles also often signal a start of a channel, like in a spike & channel trend.
In a Channel
there is a version of a XT in a channel called CXT – a channel expanding triangle, which is a pullback in a channel, that takes out a minor HL/LH
you can find a lot of examples on my charts, or just use CXT from the tag list