What is the form of potential switch function used in vdw-modifier option in mdp options?

GROMACS version:
GROMACS modification: Yes/No
I have been looking for the functional form of the potential switch used in GROMACS. The one provided in the manual seems incorrect as it doesn’t hold the boundary conditions.
https://manual.gromacs.org/current/reference-manual/functions/nonbonded-interactions.html
The 28th equation (Sv) I’m looking for!