Cross-promotion

The mechanic is simple, in that each side lends the other some attention. It differs from a collaboration, where you produce one piece of content jointly, because here the audiences are exchanged and the work stays separate. It pays off when two audiences share an interest without already sharing members, a narrower window than it sounds.

Symmetry-hunting is where it usually fails: chasing partners with matching subscriber counts instead of matching audiences. Be specific in the recommendation too. Naming one video gives people somewhere to go, while a generic mention gives them nothing to act on.