Hypnagogia
The transitional state between wakefulness and sleep, in which involuntary imagery sometimes appears — including for some people who otherwise have aphantasia.
Many people experience vivid imagery on falling asleep or waking: faces, patterns, fragments of scenes that arrive unbidden. Some people with aphantasia report hypnagogic imagery despite having no voluntary imagery while awake.
This is one of the observations that pushes against a simple "imagery switch" model. Whatever is absent in aphantasia, it is closer to the mechanism of voluntary conjuring than to the capacity for imagery itself.