Summary: Design Sprints miss out on a critical data point that can make-or-break the value generated from the effort. By spiking your first day with actual user research, gathered before the sprint, you can help ground conversations with an Outside-In design approach. This hack to the Design Sprint method can mitigate against conjecture, something the Sprint seeks to eliminate from the UX design process.
All about Design Sprints- new Lean UX techniques
A 'Design Sprint' is a technique first developed and practiced inside Google's user experience group by Jake Knapp. Knapp moved into a VC role and combined forces with Google Ventures colleagues to operationalize and refine the design sprint method with hundreds of start-ups (read: it's well road-tested).