And why is he a Twenty 20 specialist over other types.
Hitting a four or a six can be done by anyone, it just requires a lot of effort.
Tailenders can't do it as they don't have the technique in their head, so it's harder and requires a more conscious effort. Having good hand-eye co-ordination or being talented means a little less effort needed.
So why do you say Afridi will be better at T20? Cause he can put in more effort in a small period of time than others? I bet Sachin, Lara, Ponting, any highly skilled batsman would be great in T20. Look at Ramprakash, one of the best batters in T20. Being strong and being able to hit the ball hard does give and advantage, but doesn't mean that only hard hitters like Afridi will be successful.
Afridi could be great in Tests and ODIs if he concentrated more, that's what makes batsmen greater in those formats.