All is turning around a simple formula:
W = U * D
where W - work, U - units (number of resources for the task), D - duration.
Task types can be Fixed Work, Fixed Units, Fixed Duration.
Most used is Fixed Work.
If you have 32h of Work and it is fixed, you need 1 person (8h working day) and 4 days then duriation. If you add additonal resource, now you have say 2 persons - then duration will be 2.
Now if you would set task as Fixed Duration and you add second person, MSP will change Work, not duration (because it is fixed). It is less intuitive, but once you have formula and mind and you know which variable is fixed, then all become clear.
Fixed units is also a task type, and guess what - the same formula applies. Can not change unit then. By changing duration to 2 you will half the work to 16. The same MPS person can only do half of the work if you ask him to half duration.