Board Thread:Boruto Discussions/@comment-33150612-20170921061511

It's pretty obvious, it's the curse marks,

in the war Obito used rods and chains with cursed marks to dampen tailed beasts

Kawaki in my opinion probably has a stronger dampening jutsu.

Cursed marks can basically be stretched (for the plot) to weaken naruto and sasuke thus the beginning of the first episode of boruto starts. 