Sotogawa

Sotogawa (lit. outside) refers to tiles towards the outside (i.e. away from 5) of a discarded number tile. When an opponent discards a number tile early, tiles towards the outside become safer against that opponent. For example, if 3-pin is discarded early, then 1-pin and 2-pin become safer.

This theory only applies to tiles up to +/- 2 away from the discarded tile. Tiles that are 3 away are suji. Tiles that are 4 away are not affected by sotogawa, and possibly become more dangerous due to Ura suji.

Theory

Assuming you are playing against experienced players, sotogawa is a bit less safe than suji, but still on a comparable level to suji. A tile that is affected by both sotogawa and suji (e.g. a 2 after both 3 and 5 are discarded) is safer than a tile that is one or the other.

The concept of matagi suji is the main reason why sotogawa is effective. Matagi suji states that an opponent is less likely to cut a 3 from "233" or "344". So, if they cut a 3, it is less likely for them to have "23" (waiting on 1-4) or "34" (waiting on 2-5) remaining; this makes 1-4 and 2-5 safer. Of those, 1 and 2 are safe "sotogawa" tiles, while 4 and 5 are not safe as an opponent could still have a "56" or "67" ryanmen. Thus if 3 is discarded, 1 and 2 are safer.

Generally, "early discards" are the first six discards of the game. If a player declares riichi early, then the sotogawa of the riichi declaration tile become more dangerous instead of less dangerous.

Sotogawa is less effective against new players, since this theory relies on players having strong tile efficiency.