The notion of a stage-gate methodology has been with us for some time. The core idea is to break any complex project into stages, and to make sure the transition from one stage to another is a deliberate decision.
The gates act as checkpoints along the way. They allow everyone to gather and check in on whether the project is progressing as planned. But more importantly, the gates include criteria that define if the team is actually ready to take the next step.
This is especially important when the transition from one stage to another will require additional resources to act on it. Some of these transitions are much bigger than others. The transition from product design to product development, for example, is a very significant gate. Once we start building product, things can get expensive fast, and it can be very hard to reverse course. Think, for example, of the concrete pour in the foundation of your new building. That step is expensive to reverse.
But the gates don't have to slow things down. When done correctly, the stage-gate process should actually speed things up. The main way stage-gates add speed is by avoiding costly design mistakes that we have to fix later.