Many of our bulb holders have an interior locking mechanism to ensure that they are not inadvertently opened when changing a bulb etc.
This mechanism tends to be a small metal or plastic tab on the inside of the bulb holder located to one side of the connection terminal which will make contact to the inside of the twistable section of the holder itself preventing it from turning.
ALWAYS MAKE SURE THE POWER IS ISOLATED BEFORE INSTALLING, CHANGING A BULB OR UNDERTAKING ANY MAINTENANCE WORK ON ANY ELECTRICAL ITEMS.
This tab should temporarily be pushed or bent towards the inside of the bulb holder to allow the unit to be unscrewed and can gentle be pushed back into place once the work is complete however this is not always necessary.
We recommend that any fitting or installation is done by a qualified electrician, do not risk installation yourself if you are at all unsure.