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. (Among other things.)
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 players get very good at applying techniques within that narrow range. Olympic judoka who have particularly notable groundwork include Flavio Canto, Jimmy Pedro, AnnMaria Burns (now AnnMaria de Mars), and her daughter Ronda Rousey.
The other significant rule is that the match will be paused and restarted standing if there is a lack of progress in groundwork. Application of this rule varies widely across different referees and different levels of competition. For some referees a stand-up could be triggered by five seconds of no movement, despite a choke or technique being applied. For others, up to twelve seconds of inaction is acceptable, and significant attempts at turnovers, pins, chokes, armlocks, or transitions are enough to allow groundwork to continue.
There are a number of reasons that groundwork is frequently nowhere to be seen in judo matches at all levels.
Many modern referees allow very little time for groundwork before standing the players up to restart the match standing. Therefore, many elite judoka simply prefer throws to groundwork. It's common for these players to work for the referee's stand-up instead of a choke or armlock, except for one or two scenarios for which they have techniques they are very confident in.
Many players also focus their judo training time on throws instead of groundwork. This means they might eschew groundwork simply because they might end up in a bad position against a player better versed in groundwork. Because the rules allow for stalling on the ground, which leads to a referee's intervention, this is a successful strategy.
It's also common for judoka to view groundwork as more tiring or time-consuming than stand-up work. Many players avoid groundwork for that reason.
Outlawed and uncommon submissions
From the perspective of modern grappling in the vein of Abu Dhabi Combat Club or NAGA, more submissions are illegal than legal:
- All leglocks are illegal, including heel hooks, straight ankle locks, kneebars, toe holds, and calf crushes.
- All small-joint manipulations are illegal, including finger locks, wrist locks.
- Bicep crushes are illegal.
- Both blood chokes and air chokes (attacking the arteries and attacking the trachea) are legal, but neck cranks are not. In addition, one cannot touch the "mask of the face" of one's opponent, so smothering and many wrestling-style cross-faces are illegal. The triangle choke is explicitly allowed and taught in judo (as the sangaku-jime). However, some referees will disqualify a BJJ-style triangle choke finish where the head is pulled down, on the grounds that the neck is being strained.
- Shoulder locks are not allowed, but since there is a good deal of overlap between elbow locks and shoulder locks, and it's tough to tell as a third party which one is being affected, they are often allowed by referees. All forms of "arm entanglement" locks (ude garame) are explicitly allowed and taught in judo, making Americanas, key locks, and Kimuras legal. "Chicken wings", where the arm is forced up the back in a Kimura-like fashion, are not legal, since they more obviously attack the shoulder.
- Straight elbow locks are the most popular submission in judo, and are definitely legal.
Back mount is a particularly interesting case. Since there are no points awarded for hooks, and there is intense time pressure to advance to a pin, choke, or armlock, the calculus for back mount is decidedly different than it is under Brazilian jiu-jitsu or mixed-martial-arts rules. Instead of holding the superior position and waiting for a good choke, it is generally a better strategy to explosively attempt an armbar or collar choke. This is because naked chokes often take longer to set up in the gi, it doesn't help the judoka to stay on the back, and there's virtually no down side to losing the position if the submission attempt fails. Other players, instead of going for the hail-Mary choke, will go for a turnover to a pin, or simply stand up.