What it means
Healthy trends have impulses that are longer and faster than the pullbacks between them.
In practice
Impulses show where the real buying or selling is. Traders measure a trend's health by comparing them with the pullbacks: strong trends cover ground quickly in the trend's direction and give it back slowly.
The origin of an impulse is also where traders look for entries later, since the orders that started the move may still be waiting there when price returns.
What to watch for
Entering in the middle of an impulse feels safe and usually is not: the stop is far away and a pullback is due. When impulses start getting shorter and pullbacks deeper, the trend is weakening.