Under certain circumstances it is beneficial to routinely change your demon throughout the course of an encounter. This is called Pet Twisting.
The only current circumstance is when you are using the trinket [Moonwell Chalice] and delaying Metamorphosis to line up with this trinket.The basic idea of this is as follows:
- Open the fight with the Felguard and use it's Demon Soul + Felstorm, along with [Moonwell Chalice] and Metamorphosis.
- Soulburn a Felhunter once Felstorm has completed.
- When [Moonwell Chalice] has ~6 seconds left on it's cooldown begin to hard summon a Felguard.
- Use the Felguard's Demon Soul and Felstorm, along with [Moonwell Chalice] and Metamorphosis.
- Soulburn a Felhunter once Felstorm has completed.
This continues until the fight is over, or until you have no soulshards left, in which case you will want to continue to use your Felhunter.
The reason we do this is because of the way the Felguard's Demon Soul stacks with Metamorphosis and [Moonwell Chalice].
http://elitistjerks.com/f80/t110366-demonology_cataclysm_4_3_release/