What does it mean to find a symmetry which induces the transposition $(1,2)$ of the vertices of a Tetrahedron?

Question

I get that this would mean we would just switch the $1$ and $2$ vertices, but how is it a symmetry that way? What happens to vertices $3$ and $4$?

This is on a tetrahedron where the top vertex is $1$, and then from left to right, the vertices are labeled $2,3,4$.

Answer 1

This type of symmetry is sometimes known as a reflection. Imagine a mirror in the plane through the line between points $3$ & $4$ and through the midpoint of the line through $1$ & $2$.

The vertices $3$ & $4$ are left fixed by the symmetry in the mirror, whereas, as required, $1$ & $2$ are swapped.