They usually drop them when they feel threatened...although sometimes due to traumatic injury from cage funature or attacks from other geckos they can be injured for quite a while and never drop it at all...
Do you think your gecko's tail is going to fall off?
Well i kinda pinched the tip of his tail with the tweezers thinking it was the mealworm i put in his tank. But it wasn't hard because even if it was the mealworm i wouldn't want to try and kill it.