Love it! But what's the 'd gesture'? – Dolphin Paradise: Wild Friends Review

I really love this app, it's quite amazing for people who love marine life! But I'm stuck on one quest, they ask me to perform the 'd gesture' to make my mermaids dance. Yet, nowhere can I find an explanation as to how to perform this 'd gesture'. What do I have to do to use this?
Review by taylormusic4ever on Dolphin Paradise: Wild Friends.