This is dreadfully elementary but I cannot find the answer anywhere in the Reference.

Loading articulated tutorial 3d assemblies into SolveSpace works fine. However, when I try to move the models to try out constraints, the entire model moves instead of just the part I'm trying to move. I've tried to 'tack down' a stationary part of the model without success.

I've clearly missed something quite basic, but just can't find it...