Sounds like someone gave you very inaccurate information. Allow me to give you the correct rundown.

First of all, in Red, Blue, and Yellow, there were only five HMs. You could never get a sixth HM, even by using a cheat device such as a GameShark. It simply did not exist.

In Gold, Silver, and Crystal, there were seven HMs rather than five. The additional ones were Whirlpool and Waterfall. Whirlpool was HM6, and it allowed you to pass through whirlpools when in the field. (In battle, it was a weak move that dealt damage at the end of every turn for 2-5 turns. Sort of like Fire Spin, but with the Water type.)

In Ruby, Sapphire, and Emerald, HM8 was added (Dive). Also, HM6 was changed from Whirlpool to Rock Smash. Rock Smash was a fairly weak Fighting attack in battle, but in the field it allowed you to shatter rocks. (Hence the name, Rock Smash.)

I'm not sure where you got that piece of info, but it must've been a hoax. Frankly, it sounds a lot like the rumor of "Mewthree" back in the day, which I fell for myself. But anyway, there's the scoop on HM6 throughout the Pokemon games.

Oh, and before I forget, welcome to TPM! I hope you enjoy your time here.