The anatomical characteristics of the human body cause it to function in a crisscross manner. The arm-and-shoulder mechanism on one side links diagonally through the torso mechanism to the hip-and-leg mechanism on the opposite side.
Think about what you're doing while walking, running, punching, throwing, and batting. Such cross-body linkages are foundational to human functioning and a big part of athletic movement.
One type of cross-body movement involves rotational core exercises, such as weight-plate chops shown here.
How many Mondays have you been to the gym, anxiously awaiting your chest workout, only to find that you have to wait 45 minutes just to get on a bench? It seems that every guy in the gym trains his chest on Monday; it's a foregone conclusion.