How do you build a convention? Well, first you look for something artificial you need to insert into your system, then you look for an unused (or seldom-used) bid that you can assign that meaning to. Seventy-some years ago, Easley Blackwood got tired of going down in slams missing two aces, and noticed that almost nobody ever bid 4NT. He put those two things together in what is now the most famous and widely-used convention of all time.
When your partner opens a weak two and your right-hand opponent doubles, what does a redouble mean? Generally, a redouble shows a good hand and invites partner to make a penalty double if he has length and strength in the opponents' runout suit. But when partner opens a weak two, he's specifically saying he doesn't have length or strength in any of the side suits! So, the redouble can be used as something artificial without giving anything up.
So, many partnerships play the modern form of the "McCabe Adjunct" over their weak two bids.
Originally, McCabe was a convention used in noncompetitive auctions. When a weak two bid was opened, responder would bid 2NT "McCabe". This forced opener to rebid 3C, which responder would either pass or bid 3 of another suit to play. As originally written up, this wasn't terribly useful.
Eventually, someone adjusted this to being on only over doubles. The modern McCabe Adjunct is:
XX = relay to 2S, to sign off there or bid 3C or 3D
New suit = lead-directing raise of the hearts
2NT = asking bid (Ogust, Feature, or whatever it is you play 2NT as over a pass)
This worked pretty well for me for years, but when I heard of the concept of adding transfers to this auction I immediately adopted (and loved) it. Here's what I play with any partner that's willing to learn it:
XX = Long spades or a spade lead-directing heart raise
2S = Long clubs or a club lead-directing heart raise
2NT = asking bid
3C = Long diamonds or a diamond lead-directing heart raise
3D = raise to 3H with ace or king of hearts
3H = raise without one of the top two honors
So here you can have your cake and eat it too-- sign off in a long suit (transfer to it and pass partner's forced acceptance), direct the lead in a side suit (transfer to it and return to partner's major), or direct the lead (or don't direct the lead) in partner's suit (transfer to it or just bid it).
One of my partners uses this over all preempts... weak jump overcalls, opening three-bids, and even opening four-bids! I think this is just as theoretically sound.