辟 is a non-general use character with a wide range of meanings, such as false, punish, crime, law, and ruler, while in Chinese (even after discounting the obvious borrowings) it can also mean punish, castrate, execute, wail, perverse, specious, flattery, decadent, remove, twist, open, develop, summon and appoint. It comprises buttocks (J note: that upper left-hand corpse-looking element), opening, hole 口, and needle, sharp 辛. Buttocks and hole clearly combine to give anus. Needle is used in its sense of pierce/ penetrate, to give anal penetration (see also vaginal penetration 商.）(J note: another story for another time; but I read the same derivation in Taboo Kanji, so this may not be as off the wall as it looks.)Evasive movement to avoid surprise buttseks. OK.
This core meaning gave rise on the one hand to a range of meanings associated with torture, punishment, which also symbolized law and authority, and on the other to meanings associated with sodomy, which when used in relation to a male partner was also a symbol of flattery. (Note that when combined with 女 it gives a non-general use character 嬖, which in Chinese means a sexual partner/ lecherous/ depraved [though in Japanese it is listed with the euphemistic meaning of agreeable person.] (J note: well, maybe. nciku defines 嬖 as
"1. verb To favor an individual. Often used to describe the favoring of someone and inferior to one's self.
2. noun A favored person. Often someone considered to be lower in status than the one who favors them; a concubine or a courtier." The Wordtank defines the Japanese the same way-- kiniiri, a favourite.)
When 嬖 itself is combined with child 童, it gives in Chinese a compound meaning catamite.) (J note: I cannot prove this by zdic, though google links 嬖童 with 'lewd' and 'lascivious' hanzi often enough to make me believe it.)
In the case of 避, 辟 acts phonetically to express evade, though its semantic role is a matter of conjecture, and combines with movement 辶 to give evasive movement.
(辶 really does express movement in Japanese, even if nciku wants me to believe that the element alone means halt in Chinese.)