While the connotations
above are perfectly correct, I should like to point out that the expression is by no means criptic or recursive
- not for the Hebrew
speaker, that is.
In Hebrew, the present singular conjugation in this particular form is often - in fact almost always - a noun as well as a verb. This allows for the creation of many common nouns from verbs: the Hebrew words for teacher (more), for example, also means teaches, and I suppose in common translation would be put in English as "he who teaches". There are other Hebrew idioms that make use of this structure, such as mevin davar - he who understands a thing, and yode'a inyan - he who knows a business, or his business; both describing a person of experience or education. (The ha at the beginning of hamevin is the Hebrew definite article, or "the")
So (ha)mevin in this context means rather "the understander" than simply "one who understands". It's a bit clumsy in English because it doesn't have nouns of this type (Gritchka kindly informs me that the technical term is active participle or noun of agent), and a concept like this would have to be expressed in several words: "a clever person", "a man of intelligence" or something to that effect.
Seen in this light, hamevin yavin is more of a banal truism than a recursive quip, although it is indeed often used to justify a criptic statement, or obfuscate instead of illuminate.