Plank shoulder taps are an advanced variation on the classic plank position and will work your core as well as your shoulders and chest.
To perform this exercise, start in a press-up position and make sure your hands up directly under your shoulders and your feet are in line with your hips. Keeping your hips as still as possible, lift one hand and tap it on the opposite shoulder. Then repeat on the other side. Keep repeating this while still keeping your body as still as possible and squeezing your core tight. This will work the transverse muscle across the core and will help to improve your balance and co-ordination.
Take this exercise slow and steady, and make sure you complete each repetition as perfectly as possible.