Whether it is on hit or damage the point remains. The field negates the hit in full, the weapon never touches you to do damage. You could presume that the field also negates the concussive effect which the weapon causes.
The weapons English description says "when struck by..." same as Deathwatch rules which means it has to hit you, not damage. Additional the rules for force fields states in their main description:
If the result is lower than or equal to the field’s protection rating, the attack is nullified and has no effect on the protected character (although the attack may have an impact on the character’s surroundings or other nearby characters, such as weapons with the Blast Quality).
Basically meaning that you shrug it and any negative effects off from it entirely. So if you are hit with a concussive weapon, even though it's enough to trigger the concussive special rule, the rules for Force Fields overrides. Of course at GM discretion they could ignore this depending what struck the character and the type of forcefield.
Edited by Calgor Grim, 16 February 2014 - 02:54 AM.