Shou Puerh, or Ripe Puerh, has the same processing as Sheng Puerh up until it undergoes accelerated fermentation, called wo dui (渥堆). Wo dui looks to imitate the aging process by altering conditions to dramatically increase the rate of bacterial and fungal fermentation. This process is said to have been invented by the Kunming Tea Factory in 1973; the process was then improved by the Menghai Tea Factory, and large scale production began in 1975. Wo dui is essentially wet piling, in which a large amount of Sheng Puerh (at least ~1 ton typically) is piled together and fermented over 1-2 months. Ideally, the temperature, humidity, and growth of Aspergillus spp., an important fungus necessary for high quality Shou Puerh, is closely monitored. Improper fermentation is common, and the result is Shou Puerh that essentially tastes like composted tea.
Sheng Puerh often takes 20+ years to ferment over time, and Wo dui aims to develop a similar product in about a year. It is important to note that Shou Puerh will never be equivalent to a fully maturated Sheng Puerh, and there are many scholarly articles on the subject. Simply by drinking, it is very easy to differentiate Shou Puerh from aged Sheng Puerh. The secondary metabolites are fully maintained in Sheng Puerh, and the maturation of these is one reason why Sheng Puerh will develop more complexity over time than the same material made into Shou Puerh.
A brief aside on the tea market and economy: for many reason, typically the best quality material is made into Sheng Puerh. Most agree that for wo dui to be done properly and efficiently, a large amount of material is necessary. Even for experienced producers, there is a significant risk of complete or partial failure (either a ruined product or a low quality Shou Puerh). The high quality Sheng Puerh already has a very high value in the marketplace, and for a producer to risk using high grade Sheng Puerh to make Shou Puerh they typically have to have a non-monetary reason for the endeavor (i.e. they want to produce and sell a high quality Shou Puerh). The wo dui process also tends to reduce the final weight of the product by at least 20%, so this factor in isolation will reduce the value of the tea by 20%. That said, there are very high quality Shou Puerhs on the market, they are just very rare.