What does “move on an initiative of X” mean?

Question

The Young White Dragon (Monster Vault page 67) has the following trait:

Instinctive Rampage

On an initiative of 10 + the dragon's initiative check, the dragon can move up to its speed as a free action. The dragon can move through enemies' spaces and gains resist 5 to all damage during the move. Each time the dragon enters an enemy's space for the first time during the move, it can use claw against that enemy. If the attack hits, the target also falls prone. If the dragon cannot use a free action to take this move due to a dominating or stunning effect, then the effect ends instead of the dragon making this move.

How should I play it? I don't understand what "The dragon can move on an initiative of 10 + the dragon's initiative check" means.

Answer 1

I don't have the Monster Vault but I'd say the description is pretty clear.

From my reading it means that the dragon gets to take the described free action 10 counts before his own initiative comes up in a round.

For example, if the dragon had an initiative check result of 21 he'd get to make his rampage attack when initiative 31 comes up.