In the episode Sonic Rainboom, Rainbow Dash was tutoring Fluttershy on cheering for her for the Young Fliers Competition. She began with teaching Fluttershy the Elements of a Good Cheer (loss of control, screaming and hollering, and most importantly, passion), and then asked her to prove her worth and show her a good cheer. However, being Fluttershy, her cheers were quiet, soft, gentle, adorable etc. (yay is written in the lower case to emphasize this). Dash told her to retry a few times, but to no avail. On her last attempt, she took in a huge lungful of air, and then released it in an earth-shattering "yay". And by earth-shattering, we mean nothing-shattering.