A weak two-bid has two purposes. One is that it is there to upset the opponents (a satisfying part of the game) and the other it is to make it clear to partner what our hand is. In this last respect a two-bid is a LIMIT BID. It tells partner within fairly narrow limits what we have and allows him (or her) to select the final contract.
We should be careful about what we have when we open a weak two. Generally speaking it shows 5-9 points (with 10 points and a good suit we'd generally open at the one-level).
Our suit should be good; we shouldn't have an outside four-card major (otherwise we'd risk pre-empting our side out of the best contract). We shouldn't have a void on the side, either.
Now, it is important to note VULNERABILITY and POSITION at the table when pre-empting. If partner is a Passed hand, anything goes. Hmmm. Make that "you can loosen up a little".