The better way to explain the violent swing of expression with emotions in people with AS is to think of a standard light switch.
It's either ON or OFF.
Likewise, if someone with AS is not sufficiently stimulated by an emotion, that person tends to show no reaction. This does not mean that person isn't feeling anything, but instead, they are not stimulated by that emotion enough to respond to it.
If someone with AS is sufficiently stimulated by an emotion, that person tends to react as if they are feeling the emotion with great intensity.
This is why people with AS struggle with showing APPROPRIATE response to situations.
For many, as they experience an emotion state more and more and learn about different levels of appropriate response, it is possible to learn more varied levels of reactions, but their reaction has to be thought out...it rarely is the product of unconscious processing.
NTs go through a similar situation but develop mastery of emotional processing at a very young age while people with AS generally do not.