How does a grenade work?

[mohavesam]

Not an expert, but does not "pulling the pin" just allow the handle to be released that then fires the primer that lights the fuse that sets the explosive off about 3-4 seconds later?

You're thinking of a US made grenade. I think the one in the courtroom was probably a Paki designed and manufactured grenade which most likely had a different mechanism. Might even have been a homemade gunpowder one, which would account for the apparent low explosive power.

Yes. Too many assumptions being made here. A US MIL-STD (M67-type) grenade would have a DRN-kill radius of anything inside five meters, and a "severely shredded" radius out to 15M (45-50 feet in any direction). I highly doubt the subject grenade as US, as there were no deaths reported (in a closed room).
No grenade is a nice grenade.