In this paper we present InstructiBlocks, a system for constructing ambiguously defined physical-digital models, with the role of supporting the task of early stage ideation and design space exploration. Construction blocks tagged with RFID chips allow digital labelling with the build instructions. As each label only gives partial information on how to build the model it is possible for the designer to create a wide variety of models around the particular constraints that are present in both the physical blocks available and the digital instructions contained within them. In this context ambiguity is enabled by the digital instructions and there is no master copy, rather the state of the physical-digital model is determined by the interplay between the user, the physical blocks and digital instructions. In this paper we ask: Can an ambiguously defined model lead to greater design ideation and design space exploration? Are there benefits to 'humanactuated' interfaces, where a physical-digital model is constructed manually?.