Stance dancing is the art of changing stances briefly to use abilities from different stances. Because not all abilities are available in all stances, there were many situations where even tanking warriors will be required to change stances.
To see a protection warrior to do this is now rarely (if ever) observed since because of a warrior talent review in the past, there are very few abilities that generate much threat require another stance. It may be more common to see an Arms warrior do so, as they may be most likely to stay in Battle Stance, but switch to Berserker Stance in order to use Pummel.
As always in the past, typically warrior tanks would stay in Defensive Stance as much as possible. That stance generates the most threat and affords the most protection. However, in many boss fights, warriors were almost required to change to Berserker Stance at least briefly to activate Berserker Rage (which no longer requires a specific stance) in order to avoid fear or to generate enough rage to stay on top of the threat list.
Stance dancing tanks rely heavily on the Tactical Mastery talent.
Tips and Tactics
Rage generation is a key concern of a Tank spec warrior. Sword and Board tanks do not generate significant amounts of rage. Ideally a Tank wishes to use Shield Block, Devastate or Sunder Armor, Thunder Clap, and Demoralizing Shout as quickly as possible to generate significant threat and debuff enemy melee damage. This requires 46-55 rage depending on talent distribution and is difficult to generate cold using a one hand weapon. Using a stance dance a Tank Spec sword and board Warrior can easily generate the required rage. The typical rage generating stance dance sequence was Battle Stance, Bloodrage, Charge, Thunder Clap, Berserker Stance, Demoralizing Shout, Berserker Rage, Defensive Stance, Shield Block,and then Devastate/Sunder Armor. Macros can be employed to complete this sequence in under 5 seconds. For best results, a Stance Set addon would be used to automatically equip a slow two-handed weapon when in Battle Stance and Berserker Stance then automatically equip a sword and board when you switch to Defensive Stance.