The Spirit of Battle is a consumable item that can be won on Treasure Hunter. When used on a Slayer mask or its helm alternative, it will reset their kill count, which will reset the benefits that the masks give. Using the Spirit of Battle on either the mask or helm will consume the item. A mask which has yet to reach the kill count to convert it into a helm provides additional Slayer experience, bonus damage, and a double-drop every 10th kill. A slayer mask/helm that has already reached the required kill count provides no such benefits, which is why the Spirit of Battle is a helpful item.
These monster-specific slayer masks/helms provide additional benefits not affected by the Spirit of Battle.
Although it is possible to use the Spirit of Battle on a mask that hasn't reached the required kill count, it would be wasteful to do so.
Resetting a mask/helm with this item does not allow a second slayer task to be forcefully assigned within the recharge period, nor does it allow extra teleportation before the daily reset.
Slayer masks and helms