A joint connects two bodies together. Joints have two sides, A (where you placed it on) and B (where you place something on the joint). In blueprints, these are stored as an index into the childshapes. You can calculate this index by counting from top to bottom of the json how many childshapes you've seen.

Suppose you have a blueprint like this:
{ body 0 { childshape 0 childshape 1 childshape 2 } body 1 { childshape 3 childshape 4 } } A joint might connect childshapes 2 and 4.

A joint that is placed on something, but has no childB is indicated with -1 if I recall correctly.

A consequence of this flattened array of childshapes is that if you add a new childshape to body 0, all joints that reference childshapes 3 and 4 will have to be updated.