system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is there a difference between your heart beating fast and beating hard?

During exercise my heart will beat fast, but not very hard.
However when I abruptly wake up from a nightmare or see a person I'd rather avoid, my heart will beat very hard but slow.

Why is it necessary for our bodies to have two different methods and do they achieve different effects?
gpt
When you exercise, your blood pressure is regulated by a combination of your arteries expanding, arterioles in the muscle expanding, and heart rate increasing. For any given increase in intensity, it takes 2-3 minutes for that change to fully occur. 

When you’re at rest and under stress, your muscles don’t put out the same dilation (known as functional sympatholysis) yet your heart rate may still increase. In that instance, the heart has to pump more forcefully to overcome the blood pressure shift and maintain the same ejection fraction (amount of blood pumped vs amount of blood left in left ventricle).