For example, you go under contract in Aug 2005...the clause in the contract states that the developer must start construction or commit to starting construction by a certain date (Sept 2006). If it is after that date and the developer has not committed to starting construction or begun construction then the contract could be void...unless he has an addendum or a new contract signed by you extending this time.
I have personally seen this happen several times over the last several years. Pre-construction sales obviously take longer than they once did and this is a way for a developer to not be able to keep you under contract forever. Most developers will not commit to build unless they have enough pre-sales to feel comfortable about moving forward.
As always, get expert advice from a real estate attorney!!
Hope this helps!!