In Yukon MDX, you can crossjoin two sets by simply enclosing them in parentheses. In a way, it's like using the CrossJoin function without using the word. In other words, you just list the sets as if they were parameters, but leave the function name off.
But that's not really what's happening. Parentheses form a subcube. A subcube is very similar to a crossjoin, but with slight differences. With the Crossjoin function, you can't have the same hierarchy appear more than once, but with a subcube (that is, with the parentheses alone), you can use a specific member of one or more hierarchies, and then use the Tree(
Thus, the expression Crossjoin([Bikes],Tree([Product])) is invalid, because the Category attribute appears both in [Bikes] and in Tree([Product]), but the expression ([Bikes],Tree([Product])) is valid, because the subcube operator just "fills in" the unused hierarchies.
One interesting thing about the subcube notation is that it is identical to tuple notation, except that as a tuple, you use single-members instead of sets. The intriguing part about that is that a tuple really is the crossjoin of individual members, so the notational similarity really does match an underlying semantic similarity. Form follows function. The Bauhaus architects would be happy. (See, I learned something in Munich.)