Other MEP elements (e.g. Bends, Junctions, Terminals) have connections whose geometries are fixed. You can start a new Routing operation from any of these connection ports. To start Routing, click on a connection port: Guide Lines appear corresponding to the standard bend angles. Click along the Guide Line you want the Route to follow.
You cannot place the newly routed element at an angle that would require you to truncate the existing MEP element.
In the 3D window, Guide Lines are not available. Instead, begin routing by pressing Shift to constrain the first placed element to an orthogonal direction. If you want the first routing element to be placed at any other angle, enter the desired angle value in the Tracker.