I’m trying to unscrew my toilet seat but it seems to have been fitted with this plastic mechanism, which doesn't seem to want to be unscrewed.
Do I need a special tool to remove it?
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That is called a blind jack nut, jack nut, blind collapsing nut, etc.
They are typically used when you don't have access to and/or don't have the tools to hold onto a nut on the backside of whatever you're putting it into. As you turn the bolt the wedge will move along its length, moving into/out of the split nut which adds/removes force that pushes the split nut apart. In the non installed position the wedge is at the end of the bolt so there is nothing in the split nut to force it apart & the entire assembly can be pushed/pulled through a hole.
In your case the bolt is accessed from the opposite side. There are usually caps covering the bolt that can be gently popped off/open using a small screwdriver. You will most likely need to grab the entire assembly from below with fingers, a screwdriver, or pliers to keep it from rotating as you turn the bolt. Once it breaks free the nut will likely be incredibly easy to hold as you unscrew the bolt.
That is a collapsible nut that fits into various size holes.
As the comment says, undo the bolt from the top - you may have a plastic cover to remove. But no image so my "goto" is usually a small screwdriver - just don't stab a finger with it.
Note that if you keep tension on the bolt head by pulling on it as you unscrew it then the "nut" will stay located in the hole - may drop after though... Can help if getting to both sides at the same time is difficult.
The other answers aren't wrong, but may be optimistic.
On mine, the heads weren't accessible from above - the chromed metal covers weren't going to come off with any reasonable force. I managed to unscrew the whole seat by using mole grips (AKA vise grips, self-locking pliers) on the exposed threads, and used the same method to refit it. I could have tightened them up without removing it, but was curious about the mechanism; it looks like the caps on mine clip on too securely to ever be removed.
Had I thought of it in time I would have filed a pair of flats on the end of each bolt while I had the whole seat removed, so I could use a spanner next time.
You access the top of the screw. It probably has a philips head on it.
The means of removing the seat varies. I am not an expert on this but I happen to have seen a couple of these recently.
One had a button on the bottom of the lid at the centre - with lids upright. Depress button (well it lifts up away from the floor), lift lids gently and the lids come clear.
In my own place the lid assembly lifts straight up - lids upright. HOWEVER they have a detent and I couldn't move them by lifting with what seemed reasonable force. They popped off individually at each side when levered gently upwards with a 6 inch long screwdriver.
The lids lifted clear and then there were then small circular covers on each side that had to be lifted off to expose the screw heads.
In both cases I was baffled for a while and tried this and that before figuring it out. I even had fitted the latter ones myself a few years earlier and it was very mysterious but I was certain that the lids came off the top somehow to reveal screw heads.