Assembly: RevitAPI (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0)
A rebar element's flexible geometry is controlled by several handles. The shape of the bar is controlled by a handle at each end of the bar (blue circle controls) and a handle each edge (blue triangle controls). Another handle is used to control the location of the plane in which the rebar lies. An additional handle controls the length of a set of rebar.
RebarConstrainedHandles can only be constructed internally by Revit. They are available to the API by querying a rebar element's RebarConstraintsManager.