Preemptive Jump Overcalls are weak hands, showing six or more cards in your suit, nothing outside, and around 6 - 9 points, no more. They are meant as obstructive bids to upset the opponents' bidding, e.g. 1♥ (3♠). The way you can tell if it's a preemptive overcall is that there must be a jump in the bidding. The higher you jump, the more cards you show in your suit. You need all the honour cards concentrated in your long suit, and very few points outside that.
You take the opponents' bidding space away nicely, and make it harder for them. They may have a lot of points and be trying to find a fit. When you overcall preemptively, you’re not expecting to make your contract, but hoping that the penalty for going minus is less than the score the opponents could have made. Your safety net is your long suit. Because you might be doubled for penalty, you need to have around six playing tricks to bid like this, e.g.
This hand, with the seven good card suit missing only the ♥A, is worth 6 playing tricks all by itself. The more cards in your long suit, the higher you should jump. That’s how a preempt works.
Partner doesn’t have to bid (an overcall is not forcing), but they should raise your suit if they hold a few trumps. There’s not much point in bidding their own suit when you’ve told them you have only one place to play the hand.
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