They say that to move an asteroid...?

The earlier you do something to change the orbit, the less force is needed to deflect the orbit away from the probable strike zone. This is just simple, basic physics. Blowing up a massive asteroid will just make less massive asteroids and meteoroids that can still impact the Earth and do damage if you don't deflect the orbit of the original asteroid enough. Why would any even marginally sane person want to crash an asteroid into the Earth? Get professional counseling A.S.A.P.