If you're ready to build up muscular strength and endurance in your shoulders while also getting in a core workout, plank shoulder taps are the movement you need to be doing. Although this exercise is not complicated, that doesn't mean it won't get spicy.
Start in the top of your push up position. Place your hands underneath your shoulders and keep your hips down and spine neutral.
Pull your bellybutton up towards your spine to engage your core. Then, slightly shift your weight into your left hand (try to keep your hips as level as possible) and bring your right hand to touch your left shoulder.
After making contact, place your right hand back to the ground and repeat the movement on the opposite side. These arm movements should be done slow and controlled. Repeat the same number of reps on each side or see how many reps you can complete within a minute.
The key to this movement is to maintain stability through your core and keep the hips level while your arms are moving. It may be helpful to imagine that you have an object sitting on your back and you don't want it to fall off.