What is up with this fuse?
It's a "slow-blow" fuse designed to take a short high current impulse without opening, but to open for a long term overload or a brief very high current (such as a short circuit). Quite common. 

As always, replace with the same type and rating for continued protection (and no nuisance blowing).



Edit: 

The spring puts the element under tension which allows them more degrees of  freedom to adjust the characteristics. I believe the black thing is a resistive element. See more info in US2386094.