http://www.youtube.com/watch?v=1jBBZk_7EWE
I don't know how SSBM Smash DI truly works.
Ness is almost "invincible" when he is in that frozen state.
The reason why the jab or dair didn't effect Ness is because it hit him in the frozen state. It would need to make contact after that state, but it's impossible to hit him!
If you have good eyes then you will see that the Samus on the left performs a D-air on Ness. It makes contact (well normally since the hit box clearly hits him) but doesn't effect him whatsoever.
It's here:
![](http://img376.imageshack.us/img376/8479/mplayerc200611151514598rd9.jpg)
This is a hit, the hit box is greater than Samus' arm. Much greater in fact.
This is because there is a slight period of invincibility where no matter what, it is impossible to hit the character as they fly to their death. This only happens with extremely powerful moves. The period of invincibility ends quickly, but because powerful moves send the character so fast they die before their invincibility ends.
It's not really invincibility... I just mean it is impossible to make contact with the character... They become like a ghost...