This is a simple agreement to make a preemptive bid after finding a fit.

Opener bids a suit (usually a major), responder raises only to the two level, and opener reraises to the three level to preeempt.

Opener MUST have more than 5 cards in the suit.

If opener wishes to invite to game, opener bids another suit at the three level, or an artificial 2NT, if possible.
If you use this convention, recommended is that new suits by opener not promise a four card suit.

In the auction:

1 Pass 2 Pass ?

Hand 1:	xx	AKJxx	KJx	Jxx	Opener has only 5 trump and should pass
Hand 2:	xx	AQTxxx	KJx	Qx	Opener, with six trumps and a minimum, preempts with 3
Hand 3:	xx	AKQTx	KJxx	Kx	Opener invites with 3

After the 3 pre-empt, the opposition risks more to enter the auction with 3 or 4 of a minor.