The "Special Attack" weapon proficiency is not a proficiency in and of itself, but is a type of weapon proficiency. Ones included in the core rules are Sweep, Pummeling, Sapping, Shield Bash and specific martial arts attacks (flying kick, etc.)
The Special Attack proficiency can also be used by the player to create a player character version of a monster's special attack, based on his knowledge of weapons, combined with experience, various non-weapon proficiencies, etc.
Special attacks are created by the player and approved by the DM, similar to the process used when a mage creates a new spell. As with a new spell, the player character may choose to share the special attack or not with other characters. A special attack can take 1 weapon proficiency slot or more.
A special attack may have as many requirements as the DM allows. In general, the more restrictive the special attack, the more powerful it can be.
Typical special attacks include things such as an extra +1 to +3 to hit a specific type of monster, using a weapon in an area where it could not normally be used, forgoing an extra attack to do higher damage on one attack and the like.
A DM may allow a spell-duplicating effect from such an attack, although the more powerful the attack, the more limited or expensive a special attack should be. It is often easier for a character to find an enchanter to make him an enchanted item to do these things than it is to duplicate them with a special attack. (I have allowed it. I allowed a ridiculously strong dwarf with a deeper (ha, ha) knowledge of caves and the like than a normal dwarf have the special ability to cause small, directed, earthquakes, including one that killed a big beastie with a stalactite.)
Some special attacks are so 'easy' (that is, with your complex set of skills) that if you have the right combination of proficiencies they are 'free' (that is, anyone who has those proficiency groups can do them) examples of such attacks include sniping with a bow, crossbow or blowgun, stabbing someone with an arrow (given to people who have both a bow and a dagger/dirk proficiency, or high level bow proficiencies), called shots with a horse lance (only allowed with Jousting proficiency and Lance or Lance specialization.) These special attacks generally begin as DM's judgement calls and end up so common (or are so limited in use) that the DM uses established rules for them. A list of such effects is in our core rules.
Nearly anything used as a "feat" in non-advanced D&D systems that is not broken either already exists in uAD&D as a Proficiency or Special Attack.
Sources for special attacks include learning them from monsters (or trying to recreate a monster's attack) and the player's imagination.