It's against the rules: look for the rule that covers "collusion." It's written so broadly that one could argue that working in a breakaway with somebody else is against the rules.

Teamwork is banned so that one teammate can't drop back from the field to help another teammate lap up. I forget who, but somebody from GB was DQed for doing this at a fairly high-profile event maybe four years ago or so.

It's harder to enforce things like decisions not to chase a teammate. Even leadouts are hard to enforce. after some egregious stuff at a high level event one comm told me all they really looked for was deliberately slowing down in the lane to block while a teammate is off the front.