Review the item:
Everyone always invites Marc to the movies, his jeans are so oversized that
he can sneak in two two-liter bottles of soda,
which he straps to his thighs, saving his friends from the outrageous
drink prices at the theater.
Which he straps to his thighs is an adjective clause that is interrupting the flow of this complete sentence: His jeans are so oversized that he can sneak in two two-liter bottles of soda, saving his friends .... An interrupter needs to have a comma both in front and behind it.