i watched andre do this grind alot but i dont understand how he does this do i need the cap in to do this ???
No. Take the caps out.
Throw a UFO, bring the Yo-Yo up, and stick your thumb in it.
My dark magic is pretty new and so am I to yoyoing, i have learnt a lot for say 2-3 mounths but I want to learn grinds but am stuck between taking them out or not, do you think i should take 1 out
I think it’s a bit like a matador trick. Do you have a hubstacked yoyo? If you do practice it on that to get the hang of it. 
It is not a matador trick. You throw a UFO bring it up and staick your thumb in the side but not on the hub thing.
Just do a ufo, stick your thumb in there. Bring your throwhand nearer and nearer to it, letting it snag and catch. You need a semi responsive yoyo to do the wind up, Andre’s DM seems a little responsive.