Then use pads to practise focus! As long as you can hit the target with enough power to knocks pads out of a reasonabley strong grip you can be assured you'll break wood. Lot less dangerous than punting re-breaker boards around IMO!
A good friend of mine broke a bone in his hand (boxer's fracture) doing speed breaks with rebreakables. Yes it was bad technique but if it had been a pad he probably wouldnt have had to wear a cast for 2 months . We have been using 'the ultimate board', they have long fingers that slip into each other and provide variable amounts of resistance, ie 1 board to 2.5 boards. They tend to last alot longer then the thin c-channeled deals. The surface is super hard though and has some sort of faux wood grain that likes to tear up your knuckles. So I'd recomend putting a leather/chamese pad over it to save your skin.
I have used rebreakable boards before. But I found that the pine boards (or whatever you use) work better that the plastic for any kind of breaking. The thing with rebreakable boards was you have to hit them exactly where they are put together or they will not break, unless of course you can kick or punch extremely hard. They do work great if you are just practicing accuracy since they are the size of most boards people break. I also found that they are extremely expensive, unless you already have some. Don't know if that helped or not.
