Mathematicians have found a hidden 'reset button' for undoing rotation

This made me wonder if there are knots you can't untangle.

Yup, the trefoil knot is one

I'm not sure I follow. Every knot is defined as if you close the ends it cannot be unravelled without cutting the ends again. So the trifoil knot is included in this... but so is almost if not every other knot aren't they? Do we have "knots" that aren't mathematical? I feel like if you tie any "knot" then fix the ends together most or all of them would not be possible to untangle.

Every (mathematical) knot is one that can't be untangled, by definition.

Every knot with a cut can be trivially collapsed go a point by moving one of the endpoints to the other one through the path of the knot