It is more than simply noticing increases or decreases in keywords over time. Trends involves having a deeper understanding of content and context, and calling out notable changes with respect to a given baseline.
For example: We’ve detected a shift in the trend from people feeling as though the trend towards selfdriving cars is bad to people feeling as though the trend towards selfdriving cars is good within the following specific circumstances…
Trends might be isolated and tracked down to the neighborhood or even block level. Trends which are identical but spread, say geographically through a wide area might also be detected. Thus a trend that is widely distributed through neighborhoods and hence too weak to detect might be detected by this system.