How does Assault Fuse work, exactly?

The in-game description says that the bombs will explode on impact, unless they were falling for less than one second. From my own testing, it seems that the time the bomb must be falling for assault fuse to delay the explosion seems to be more than one second and the time it takes the bomb to explode also seems to vary. I’m not sure what’s going on here, can anyone explain how exactly assault fuse works?

It used to be as described, but I’ve had problems with it for a year or so… now I just use 0 second delay and very occasionally blow myself up as a price to pay for at least knowing the damned thing will work as I have selected!