IMO unless you have a tool like Danny showed this isn't possible with a 3/8 ratchet or small extension! The spring is just too close to the insert. Oh and its very weak and the spring is stupidly heavy!
Air con delete ftw
You push it backwards, so it lifts the tensioner upwards.
Its only cast ally, quite easy to break, if you do break it then its back to the jubilee method.