Shoulders taps is a core (Transverse Abdominis) workout, though it does deliver other benefits too. 

What's involved: Deltoids, triceps, abs (Rectus Abdominis), core (Transverse Abdominis), glutes, quads, lower back. 

Powered by: Triceps, core. 

Tip: You can increase the execution speed of shoulder taps to make it more challenging to your core.