Judo's groundwork (newaza) looks strange to someone from a Brazilian jiujitsu, wrestling, or SAMBO background. Its approach is fairly unique to this particular combat sport. Why? Because rule-sets determine tactics.
The basics of judo newaza
Other than throwing the opponent, one can win in judo by pins, arm-locks that attack the elbow, and chokes. Some ...
Chokes are allowed, and elbow attacks are allowed. Everything else is not allowed.
There is a bit of a grey area when it comes to "key locks" or ude garami, which can put the shoulder in peril, but it also attacks the elbow so is allowed, as well as the guillotine choke which while a choke can also be a neck crank, so some referees will prohibit it.
Yes, there are other constriction techniques. These are a few examples, not meant to be exhaustive. As a philosophical matter, I think you should always be looking to further break uke's structure and constrict their movement and breathing; this is maximum efficiency.
yoko shiho gatame: Tori is on the side with extended legs, one arm around the head, and ...
According to the Sport and Organization Rules of the IJF (2019), the following applications of katame-waza are illegal:
Joint-locks applied anywhere other than the elbow (i.e. neck, shoulder, wrist, hip, knee, ankle, finger, toe locks etc).1
The leg-lock ashi-garami is explicitly illegal.14
Exception: The shoulder-locks ude-garami and ude-...
There are a few positions where you can finish multiple submissions simultaneously or nearly so.
If you look at the triangle as a position, then a lot of other submissions present themselves as co-attacks. The triangle-armbar is the classic; it's often unclear which one does the most work in any given case. The triangle-Kimura is also great fun.
Yes. From triangle (sankaku) positions, there is the standard choke with the legs as well as the option to apply armbars. For example, from this side triangle position, if the choke fails, you can either cross body armbar (juji gatame) the near arm, or Kimura (ude garami) the far arm.
While the nutcracker analogy is useful, there is a subtle difference:
with a nutcracker, the stress is applied to the nut
with a compression lock, the 'nut' (limb you are using as a fulcrum) is supposed to sustain the stress and redirect it to the 'axle' (the opponent's joint)
Some basic newtonian concepts for improving the efficiency of your compression ...