This is a complicated subject but the short answer is (a) the requirements for patentability are no different in a provisional or regular (nonprovisional) patent application and (b) when you have to file due to a potential bar on patentability, you file what you can as fast as you can.

The provisional is often sold as something that you can send in a short, scribbled out description and then after a year, you can write out all of the “details”.  In short, this is not true but it is often propagated by people who do not understand the the US patent system and laws.  Even the USPTO pushes this but then they turn around and reject your application for not being enabled.

Keep in mind, the provisional was only created as an artificial construct to put US inventors on an equal plane with foreign inventors.  When the US changed to a 20 year form filing date system, the Paris Treaty allows a citizens of other countries to file a patent application in their home country (say, Germany) and then the inventor has one-year to file in the US.  Such an application will then get a term of 20 years from the US filing date (effectively 21 years from first filing).  US citizens complained that they only got 20 years in the US so the provisional application was created so that a US inventor could file a provisional, wait one year, file a nonprovisional and effectively get a term of 21 years.

The FORMAL requirements for a provisional application ARE less than a nonprovisional.  The filing fee is low cost ($125), you do not need many of the formal papers like a declaration, and technically you do not need claims — although for a variety of reasons you should always put at least one claim in the application.

However, the provisional application did NOT change any of the requirements for obtaining a filing date which include all of the elements of sections 101, 102, 103, 112, etc.  Particularly, in order to achieve the benefit of its “priority” filing date, the provisional application MUST comply with the requirements of 35 U.S.C. § 112, namely: adequate disclosure of the invention, enablement, and best mode.