
Ok this is really easy.
first he uses cinema make up and special effects very well. what I believe he has done is, keep a packet of Polos in his pocket just ready for the trick. Also behind his back (possibly in the label of his T-shirt) he has another half sucked polo (waiting in the wings)
He asks someone to pick a Polo out of his packet (any Polo)and put it into his mouth.
He then takes off his necklace (metal) and ties it around his nick the both ends behind his head. While sucking on the Polo in his mouth and eventually swallowing it, He slips the Polo behind his head onto the necklace. without letting it fall down the necklace,
Now this is the amazing bit, what looks like him chocking himself with his necklace, he is actually squeezing it into a prosthetic neck made by a Hollywood special effects lab, stuck to his neck at the front. and also easy enough that it will break but come back together.
Or he just dug it really deep into his skin.... Im sure he uses fake skin at some point though.
As he wisps away the necklace from his neck, He lets the half eaten polo from behind his head, fall down his necklace from his left hand. and appear that it has just come out of his neck. The whole movement is done so fast you cant see the Polo appear from anywhere.
but you do see it wiggle from side to side as it comes to a stop in the middle of the necklace, meaning it had to come with force from one side.