The ju in juji-uke refers to the number 10, which is rendered as 十 in Japanese. You can see where this is going. The crossed arms in the "block" are represented rather conveniently by 十. Much confusion arises over how the block is applied. Tradition would have you cross the arms simultaneously as is done in kihon and kata. That tradition, I argue, would have you struck square in the face. A worthy opponent would fake and get you to commit everything, like the Maginot Line, to a defense that is easily circumvented.
Consider what the video clip above offers, a clever defense against a jab-cross combination, that utilizes position, control, and leverage to defeat the attacker. You are not merely waiting for the blows to rain down on you as you cover: you are setting up your opponent to walk into an ambush. If you are wondering where the crossed arms come in, the defender does cross arms, though not simultaneously. In the basic application, it's the attacker's arms that get crossed. That's art. That's a better juji -uke.