cliff notes on strength D? Thats not ringing a bell in my (limited) rule book knowledge.
Essentially, it is a strength of weapon in a class of its own. Armor saves can't be taken against strength D, but invuln and cover saves can, when applicable. Instead of rolling to wound, you roll a d6 on the D table:
1: no effect, on vehicles or non-vehicles
2-5: vs vehicles inflicts a penetrating hit that takes away d3 hull points instead of 1
vs. non-vehicles they lose d3 wounds instead of 1
6: vs. vehicles causes a penetrating hit that inflicts d6+6 hull points instead of 1, no saves of any kind are allowed (including invulns etc.)
vs. non-vehicles it causes d6+6 wounds with no saves of any kind allowed.
The catch is that (if I understand correctly) that wounds generated by the d6 table don't transfer over to the rest of the target unit. So you roll however many hits you got 1 by 1 on the D table to determine the effects on models, not the entire unit.