  The evolution of intermediate-mass and massive stars speeds up considerably
after they finish their hydrogen core-burning. Due to this accelerated
evolution, the probability to observe stars during this episode is small. In
suitable stellar aggregates, in particular star clusters of appropriate ages,
the fast evolutionary phase causes a depopulated area~--~referred to as the
Hertzsprung gap~--~in color-magnitude diagrams and derivatives therefrom. The
explanation of the speed-up usually resorts to the star's Kelvin-Helmholtz
timescale and the Sch\"onberg-Chandrasekhar instability is called upon. This
exposition challenges this viewpoint with counterexamples and argues that a
suitably defined nuclear timescale is enough to explain the fast evolution. A
thermal instability, even though it develops in stars evolving through the
Hertzsprung gap, is not a necessary condition to trigger the phenomenon.
