JUMP BID OVERCALLS:

When an opponent has bid a suit at the one level, you can jump bid to show a weak hand. However, never use the bids below to preempt over a preemptive opening; the likely outcome will favor your opponents.

Jumping One Level:

Jump bidding one level in your suit (bidding one higher than the lowest available bid in your suit) is just like a weak two opening bid. You need 5-11 HCP and a 6-card suit. Note that while you can do this in either direct or balancing seat, it is dangerous to jump bid after both opponents have bid. Thus, if your RHO opens 1H, you may bid 2S (skipping the available 1S bid) to show weak points with length in spades. However, if your RHO opens 1S, you need to bid 3H (skipping 2H, the first available heart bid) to show weak points with length in hearts.

You may be wondering why you wouldn't make a simple overcall if you have the points, and there's a simple answer: you would, in most cases. If you have a suit that is strong enough (two of the top three trump would be good) that you would be willing to rebid one level higher (overcall 1S with 2H and be ready to bid 3H later to show length), then make a simple overcall. However, if your hand is weaker, use the power of the preempt to make life difficult for your opponents.

Jumping Two Levels:

A jump of two levels is a little more dangerous. Such a bid will show a 7-card suit, but you should have a couple of points rather than the normal 0-9 HCP used for a weak three bid. Bidding a double jump to the four level (because your suit is lower than the one you are overcalling) is ill-advised without a strong point holding in the suit you are bidding as you cannot afford to lose more than one trump trick in most cases. It is better to reserve jump bids to game for self-supporting hands in balancing seat.