Unfortunately Dark Heresy doesn't really adress unarmed combat in depth and some of the more lethal techniques associated with it like strangulation and snapping parts of the spinal column.
There is a "Grapple" action but there's really not much you can do with a grappled victim except holding them down and "causing them injury" (pretty miniscule injury according to the RAW).
I was thinking of perhaps allowing a character to attempt to snap a victims neck by making a called shot grapple aimed at the victims head. The called shot penalty representing the natural difficulty in getting a good hold on someones head (which can be negated by being sneaky and attacking an unaware target of course). If the grapple succeeds and the target has become grappled, the grappler may make a grapple action (as if he or she was grappled), but in an attempt to snap the victims neck. Im not sure if one should pit the grapplers Strength versus the victims Toughness or the victims Strength, and I'll have to think about it. But if the grappler succeeds in this, the victim dies. (I think it's unecessary to measure the exact damage a victim would take since having ones neck snapped is not something you could really "heal from" if done properly).
For strangulation, im thinking of doing sort of the same thing, but using the rules described for suffocation instead. However the victim is not allowed to do the "hold your breath" option described in the rules (it doesn't matter if you are being grabbed from the back or engaged in melee combat, you wont be ready for a sudden attempt at strangulation to be able to fill your lungs and hold your breath). Also im thinking of circumvent the rules about being able to hold your breath for twice your TB rounds when being suffocated and being engaged in a strenous activity. The average toughness bonus for a normal human is 3, that would mean that a human that suddenly gets a vice grip clamped around his/her windpipe and vital bloodvessels in the neck will be able to stay conscious/awake and not suffer any injury for 6 rounds. That's roughly 24 seconds (if one round is roughly 4 seconds as per the RAW), and I find that hard to believe that you can actually manage to stay perfectly unharmed for that long with your windpipe and blood vessels obstructed (remember, the danger of strangulation isn't only to be unable to breathe, but also having the little oxygen you have in your blood be cut off from the brain).
I'd say that suffocation from strangulation will start affecting the victim after TB/2 rounds rather than the standard TB*2 rounds. Representing that the blood to your brain is being cut off due to the strangulating grip.
Im also thinking of giving bonuses if the strangler is using a tool for the job (like a garotte).
Any thoughts?