Prototyping
Within the design process being able to effectively validate and iterate on concepts is crucial when aiming to produce a refined outcome. Prototyping aids in this process as it allows designs to be tested in their use case to identify potential problems that can be overlooked during the design phase.
Prototyping is a skill I've made frequent use of when producing parts that either need to be machined or tested before their final production. These methods have included blue foam model making for ergonomic/proportion studies, 3D printing using processes such as FDM to SLA to validate designs before machining, quick and dirty prototypes to test ideas, and finally using a range of computer-controlled machinery such as mills, lathes, laser cutters, and brake presses.