There are four basic events to follow on a momentum chart. The first is a crossing of the zero line. If momentum is negative and crosses the zero line to positive territory, it could be a buy signal. This is because prices are accelerating their advance as new buyers enter the market. Note that this is only a valid signal when the line is crossed in the direction of the prevailing trend.
Trends are still more important than their derivative measure, momentum.
The second event is a divergence with price action. If prices make lower lows but momentum makes higher lows, down side market momentum is declining. In March, while the market was still falling, momentum was becoming less negative. In other words, the speed of the market was declining, indicating that selling power was weakening. Note that during the period of divergent momentum, the indicator crossed the zero line several times.