Bodystorming is a technique sometimes used in interaction design or as a creativity technique.
The idea is to imagine what it would be like if the product existed, and act as though it exists, ideally in the place it would be used. It is going through an idea with improvised artifacts and physical activities to envision a solution. This User Experience Design (UXD) technique is ideal to design physical spaces (e.g. the interior design of a shop) but can also be used to design physical products or software.
Opinions on this method
The proponents of this idea like to point out the fact that you get up and move, trying things out with your own body, rather than just sitting around a table and discussing it while having to imagine it in the abstract (as in the case of brainstorming). It is a proper user-centered design method, since it can be carried out by the designers as well as the users of the final product.