Question: I wonder what the following violin technique is called it happens in the following video at ar 4;50. It seems that it is some sort of slapping motion with the bow and also something that resembles pull offs.

Answer: 
Looks to me like the player is interspersing conventionally bowed notes with left-hand pizzicato notes. This is similar to the pull-off (*ligado*) technique on guitar, but has a quite different sound.

I only watched the clip once, but it appears that the passages that combine bowed and L.H. pizzicato are executed as follows: a note is fingered with the little finger on the L.H. and bowed conventionally; then, the L.H. little finger plucks the string it is on, sounding a note fingered by another L.H. finger, which then plucks the string again, sounding either another fingered note or an open string. This is then repeated on different strings. 

Of course, this is all executed very rapidly, giving a unique sound not achievable using a conventional pizzicato.

There are violin techniques that involve "slapping" the bow against the strings, but I can't see them in the passage you give the time of. These techniques include:

* *spiccato*, where the bow bounces on the string.
* *col legno battuto*, where the wooden side of the bow is "tapped" against the string.
It's a combination of spiccato bowing (bouncing the bow off the strings) alternating with left-hand pizzicato. 

Of course the *real* Paganini did this sort of party trick having slashed three of the violin strings with a knife, and then holding the violin upside down, if some of the stories about him are to be believed.