The final stage in production ramp before full-on mass production (MP). It should be as close to MP workflow as possible. At this stage your product is assembled at real assembly line. Factory workers get accustomed to your product, quality requirements and assembly routine. The Factory managers creates SOP (standard operation procedure). Don’t forget to send your engineers to the assembly site, since you can find a lot of bottle necks and be able adjust/improve the process before going to the MP.
Almost all startups are under the pressure to ship product to the first backers as soon as possible. So they use PVT phase (if the quality allows) as the first batch for early backers.
The pre-production stage is the most stressful and most important stage. Define your go/no-go criteria in advance. Be ready for many iterations on fixing something and repeating again. It never goes smoothly.