This corresponds to all points on the reciprocal lattice that lie within a sphere whose radius is related to the energy cutoff.
The points on the reciprocal lattice which represent the basis set will no longer correspond to a sphere, but an ellipsoid.
The Pulay stress is often nearly isotropic, and tends to result in an underestimate of the equilibrium volume.
[2] Similarly, the error occurs in any calculation where the basis set explicitly depends on the position of atomic nuclei (which are to change during the geometry optimization).
[3] The way to eliminate the erroneous forces is to use nuclear-position-independent basis functions,[4] to explicitly calculate and then subtract them from the conventionally obtained forces, or to self-consistently optimize the center of localization of the orbitals.