What does that plugin really do?

[cforce]

How does it "require" spend time and when? When i create an issue and when i edit it or even later?

[maxprokopiev]

When you change issue status.

Imagine that you have multiple statuses:
New(id = 1), In Work(id = 2), Closed(id = 3)
In plugin settings (Status IDs), write 3 and Apply.
After this, redmine will force you to set spent time, when you close an issue (i.e. when issue status is equals 3).