Looks like these guys are using the infamous Fort Troff Magnetic Nip Tuggers... one of the most intense nipple torture devices out there! [insert evil grin here...]
http://68.media.tumblr.com/624b63e8c4bf4acc3acd7a0f085e23eb/tumblr_oqjwwkri4Y1w9rdppo1_400.jpg
http://www.forttroff.com/mm5/graphics/00000001/62319_42_2.jpg
For a fun discussion of these and warnings see the following:
https://m4m-forum.org/threads/magno-nip-tuggers.91093/