The Thorn Spring is a floating object with a modern Spring on top and a metallic cup on the bottom which surface is covered in spikes. In gameplay, the Thorn Spring flips itself upside down for a short while in fixed intervals, making either its top or bottom point upward. The player can either touch them normally or target them with a Homing Attack.
When touching the Thorn Spike while the Spring-side is up, it works like a regular Spring. If the spike-side is up though, the player will take damage upon touching it. Using this gimmick properly is thus based on timing.
Sonic and the Secret Rings
Certain Worlds feature unique variations of the Thorn Spring. In Pirate Storm, there are Thorn Springs that only flip once and never turn back up. Then there is the Skeleton Dome, where some Thorn Springs either do not flip at all or are constantly flipping. The latter's movements make them virtually impossible to use.
The Thorn Springs appeared in the Xbox 360/PlayStation 3 version of Sonic Unleashed. In this game, they are only found in the daytime Action Stages. They are also not very common, only appearing in Jungle Joyride Act 1 and a few non-obligatory Acts.