Don't know how to call it... but it's not a stand alone technique.
It's a normal riff, repeated few times with a reasonable speed.
And it starts slowly gradualy increasing speed, this is called "accellerando" ... a common name in classical music.
It should be played by finger style, using 3 fingers (thumb , index & middle)... I'm sure that any experienced Flamenco or Classical guitar player can easily play it... I remember that I finished the whole song while I was still a beginner with classical guitar.
After going into elec guitar, I was interested about playing this song with a pick... You can do that riff using Sweep Picking technique... That would take a couple of days of practicing (If you're familiar with sweeping).