diff --git a/terraform/hook.go b/terraform/hook.go
index 47f17c495f..e4ad420165 100644
--- a/terraform/hook.go
+++ b/terraform/hook.go
@@ -33,6 +33,12 @@ type Hook interface {
 	PostDiff(*InstanceInfo, *InstanceDiff) (HookAction, error)
 
 	// Provisioning hooks
+	//
+	// All should be self-explanatory. ProvisionOutput is called with
+	// output sent back by the provisioners. This will be called multiple
+	// times as output comes in, but each call should represent a line of
+	// output. The ProvisionOutput method cannot control whether the
+	// hook continues running.
 	PreProvisionResource(*InstanceInfo, *InstanceState) (HookAction, error)
 	PostProvisionResource(*InstanceInfo, *InstanceState) (HookAction, error)
 	PreProvision(*InstanceInfo, string) (HookAction, error)