  The pop-stack operator of a finite lattice $L$ is the map
$\mathrm{pop}^{\downarrow}_L\colon L\to L$ that sends each element $x\in L$ to
the meet of $\{x\}\cup\text{cov}_L(x)$, where $\text{cov}_L(x)$ is the set of
elements covered by $x$ in $L$. We study several properties of the pop-stack
operator of $\mathrm{tors}\Lambda$, the lattice of torsion classes of a
$\tau$-tilting finite algebra $\Lambda$ over a field $K$. We describe the
pop-stack operator in terms of certain mutations of 2-term simple-minded
collections. This allows us to describe preimages of a given torsion class
under the pop-stack operator.
  We then specialize our attention to Cambrian lattices of a finite irreducible
Coxeter group $W$. Using tools from representation theory, we provide simple
Coxeter-theoretic and lattice-theoretic descriptions of the image of the
pop-stack operator of a Cambrian lattice (which can be stated without
representation theory). When specialized to a bipartite Cambrian lattice of
type A, this result settles a conjecture of Choi and Sun. We also settle a
related enumerative conjecture of Defant and Williams. When $L$ is an arbitrary
lattice quotient of the weak order on $W$, we prove that the maximum size of a
forward orbit under the pop-stack operator of $L$ is at most the Coxeter number
of $W$; when $L$ is a Cambrian lattice, we provide an explicit construction to
show that this maximum forward orbit size is actually equal to the Coxeter
number.
