A small fraction of them are excited enough to be able to beta-decay by emitting a delayed neutron in addition to the beta.
Delayed neutrons play an important role in nuclear reactor control and safety analysis.
A subsequent fission fragment occasionally undergoes a stage of radioactive decay (which is a beta minus decay) that yields a new nucleus (the emitter nucleus) in an excited state that emits an additional neutron, called a "delayed" neutron, to get to ground state.
Delayed Neutron Data for Thermal Fission in U-235[1][2] If a nuclear reactor happened to be prompt critical – even very slightly – the number of neutrons would increase exponentially at a high rate, and very quickly the reactor would become uncontrollable by means of external mechanisms.
The control of the power rise would then be left to its intrinsic physical stability factors, like the thermal dilatation of the core, or the increased resonance absorptions of neutrons, that usually tend to decrease the reactor's reactivity when temperature rises; but the reactor would run the risk of being damaged or destroyed by heat.
In that regime, neutron production overall still grows exponentially, but on a time scale that is governed by the delayed neutron production, which is slow enough to be controlled (just as an otherwise unstable bicycle can be balanced because human reflexes are quick enough on the time scale of its instability).