The opposites core exercise works the entire core musculature including the abs, obliques, erector spinae, and the glutes. It also challenges your stabilization and balance. This is a functional exercise, and it can be done by beginners, intermediate or even people with advanced physical capabilities.
To do the opposites exercise start in a plank position with your elbows on the ground. Your body should be in a straight line with your abs and glutes tight. Reach one arm out and above you while simultaneously doing the same thing with the opposite leg. As both extremities are outstretched, squeeze your back muscles for a second or two before releasing.
To increase the degree of difficulty do the same thing, except in a push-up position. From there you can add to the exercise by touching your elbow to your opposite knee before extending them outwards. Keep your whole core tight to decrease instability and the wobbling sensation.