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.
While I didn't notice the col legno battuto anywhere the rest of the above information is correct. Also noteworthy is the incorporation of ricochet bowing in the phrasing of the beginning of this piece. The spiccato that appears in other parts of the piece you will notice uses the middle and lower half of the bow. The spiccato used to facilitate and shape the left hand pizzicato motives uses the upper fourth of the bow helping to match its attack and dynamics with the relatively weak sound produced by left hand pizz on the violin. While the stronger spiccato allowed the bow to draw more life for each note the upper bow spiccato resulted in little slaps with the end of the bow breathing just enough life into the string for the left hand technique to continue the phrase and manage the wave in the string.