These managers find trends across a large diverse set of markets and then invest long or short based on these trends. If the markets are moving higher, they are a buyers, and if price move lower they are sellers. Buy high under the assumption that prices will move higher and sell low under the view that prices will move lower.
One investment approach is to not make any prediction. The trend-following properties of diversification should lead to a constant allocation. Some may take a more active approach and invest in a performance trend. Others will take the opposite approach. The best time to buy is when returns are low and the best time to sell is when performance is high. Be a trend-following contrarian. There is evidence that this approach works.
The contrarian view suggests that trends in any one direction do not last forever; consequently, there will come time when strong performance will be reversed or at least the return to risk will turn down. Similarly, there will periods when poor performance will be reversed because risk-taking is reduced or the period of no trends is finished. Investing in trend-followers through thinking like a mean-reverter while paradoxical does work.