I did a bit of research using our modern day equivalent: the M203 grenade launcher.
The M203 HEDP fuse grenade round does not arm itself until it has traveled 15 to 40 meters; it then becomes live and explodes on impact. The round arms itself while spinning through the air. Once it has rotated enough times, it becomes live.
So lets say the M40 grenades stay safe until one of two things happen:
1- They are launched from the tube of a grenade launcher. At which time the round arms itself (or is armed by the GL) and detonates on impact.
2- They are armed manually by removing the safety cap, and depressing the arming fuse button. After which the round detonates after a three (four?) second delay.
So how does it arm itself? Well it could arm itself the same way the M203 round does: by rotation. However it is possible the round is armed by the grenade launcher: That is what the brass primer on the base of the grenade might be for. The round becomes live before it leaves the launch tube and explodes on impact.
As for manual use: With a three second delay, the round can be thrown by hand like a modern day hand grenade. If you watch the WS version of Aliens, you'll see that Gorman presses the grenade button, Vasquez and Gorman start "hugging" the grenade, and a couple of seconds later it explodes. In this case they used it to commit suicide, but under different circumstances (i.e. being outdoors) they could have thrown it like a hand grenade.
Of course the grenades are susceptible to detonation by overheating: otherwise Ripley would have had to push all the buttons on all the grenades of her bandoleer before throwing it into the Queen's nest.
Hope this helps.
Kevin
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