Test the prototype with real live humans, and validate. The team finally gets to see live users interact with their idea,s and hear direct feedback from the target audience. Everyone on the team observes the Validation sessions: watching your users try out the prototype is the best way to discover major issues with your design, which in turn lets you start iterating immediately. In addition, the team can organize a review to collect feedback from Technical Experts or Leadership Stakeholders.
Design Thinking is an iterative and non-linear process. This simply means that the design team continuously use their results to review, question and improve their initial assumptions, understandings and results. Results from the final stage of the initial work process inform our understanding of the problem, help us determine the parameters of the problem, enable us to redefine the problem, and, perhaps most importantly, provide us with new insights so we can see any alternative solutions that might not have been available with our previous level of understanding.