The productive forces are not their legal / juridical representation.
A factory is not the shares in the holding company that claims to own that factory.
The productive forces are the objects of production and those social techniques unconnected to the specific relations of production: relations of property. For example, time and motion study is connected to sweating labour: it is a relation of production. Sorting mail faster by using shelving and ordered numbering doesn’t impose property relations. Both are forces of production, but time and motion is also a relation of production.
The forces of production in capitalism are amplified to a level of superfluity where one hour of labour can, by mechanisation and technique, satisfy tens, hundreds, thousands, billions (think software) hours of human subsistence. Capitalism relies on market scarcity to maintain price and price of labour as property relations. Under the stress of the superfluity of commodities these implode.
In concrete terms capitalism networks labour and increases its scope of command over the non thinking world. Labour is collectivised by the wage and factory producing networked collective subjects. In the London riots think about how rioters used blackberries with free or cheap messaging to organise mass theft. In Italy in the 1960s think about how the factory and supermarket concentrated workers and commodities. The concentrated workers raided and took the concentrated commodities for use.
These are historical examples, and non revolutionary, of how the expansion of productive forces (useful things and ways) have exceeded the productive relations (property law.)