A psych is a deliberate call which misstates one's hand. Its purpose is to mislead the opponents so they cannot find their best contract. However, there is always the danger of misleading the partner as well.
A psych, by definition, is not part of the partnership agreement. If anyone consider the psyching possibility into account, it is no longer a psych but a partnership agreement which must be disclosed to the opponents.
After partner opens 2♥, it is clear that the opponents has a huge spade fit and values for game. Instead of raising to 4♥ and hearing the opponents finding the huge fit, the hand responds with 2♠, which is forcing, planning to rebid some number of hearts afterwards in order to confuse the opponents that they don't have a good ♠ fit.
Opening this a strong and artificial 2♣ is referred as a semi-psych, because others expect something which is very different from this, but this is actually not a psych because it fits one of the common criteria of opening 2♣: at most 1.5 tricks short of game, and this hand actually has a game contained in it. The purpose of opening 2♣ is just to force to game, and this can be said as a non-standard 2♣ opener.
Psychic control Edit
If the partnership has agreement to determine whether a previous bid is a psych or not, by definition, the bid is no longer a psych but must be disclosed since a partnership agreement exists. However, the terms psychic control and controlled psychs are still in widespread usage.
A controlled psych is a bid, which has at least a normal meaning and an alternate meaning, which is referred as the psych. A psychic control, or check bid is a follow-on bid, used when necessary, to find out whether the previous bid has the normal meaning (not psyched) or the alternate meaning (psyched).
Here is an example of psychic control, used by Kaplan and Sheinwold: A one of major opening shows either a standard opening bid (the standard meaning) or a hand with 2-4 HCPs and 4-5 cards in the major suit bid (the alternate meaning, aka controlled psych). To test the seriousness of the bid (controlling the psych), the responder makes either a jump shift or bid 2NT (psychic control), and the opener bids 3 of the original major to indicate he has psyched, or anything else to confirm a full opening bid.
This is actually not psyching by the definition of term, but rather a set of agreements. This kind of agreements are usually classified as brown sticker and restricted to certainly level of events.