This last patch blizzard made it so ghouls have a CD that only STARTS once the current ghoul dies. So basically you cant have a ghoul, get it killed, and instantly resummon another. You'd have to wait the duration of the CD because it will only start once the ghoul dies.
The trick is in arena your pet is a valuable asset, SPECIALLY because of the ability Death Pact (Sacrifices your ghoul healing the deathknight for 40% of max hp) but the problem is after its used you cant get your ghoul back for awhile while waiting for the CD.
The trick is this, in arena summon your ghoul and then mount up! By mounting your ghoul disappears and the game believes here is no ghoul, therefore starting the CD.
What this means is because the CD will be faded once the battle starts, allowing for a free 40% heal from death pact and then you can instantly resummon your pet.
Im not sure what kind of screen shots would work in this situation, just go on your DK, summon a ghoul, mount up and watch the CD timer fade for the ability Raise Dead.