This is a business card unit which I designed independently with Jennifer Campbell who used the name Dog Bone Unit (details in model description).
Instructions on the web
You can find her instructions on this web page.
Folding a unit
Assembling a cube from units
Swallowtail units can be used to fold a cube. I believe that with units made from somewhat slimmer modules (paper with 3:1 ratio or similar), one could fold the units in half and create some spiky balls based on triangular spikes (similar to this spiked dodecahedron), but haven’t yet tried it in practice. In either case, at any vertex three units are connected to each other. The “swallow’s tail” of each unit is interwoven with the corresponding part of the other two units, creating a small sunken pyramid at the vertex. This connection is quite stable once all units are connected (since the different forces cancel each other out) but a little fragile as long as the whole polyhedron is not yet complete. The joint works due to the business card’s high stiffness. A similar connection made of regular paper wouldn’t hold. Some units made from regular paper, most notably the PHiZZ module, use a similar connecting structure, but they need an additional sheet of paper in the pyramid while in the business card version one layer less is enough.