Available with 3D Analyst license.
Use the Connect TIN Nodes 
interactive tool to establish a breakline between two nodes. The breakline will obtain its start and end heights from the TIN nodes.
The Connect TIN Nodes tool is useful when you want to override the existing triangulation and enforce a linear feature on the surface.
The benefit of using the Connect TIN Nodes tool is that the enforcement as a breakline ensures subsequent edits in the nearby vicinity will not undo the change.
You can select either hard or soft as the edge type to use when creating the breakline between the two selected nodes.
- Click the TIN Editing drop-down arrow and choose Start Editing TIN.
- Click the Connect TIN Nodes tool on the TIN Editing toolbar. The Connect TIN Nodes dialog window appears.
The cursor appears and interactively highlights and snaps to the closest valid node as the cursor is moved across the TIN surface.
- From Connect TIN Nodes dialog window choose either hard or soft as the edge type.
- To connect two nodes, click on the first node to select it. A graphic line displays and snaps to the closest valid node relative to the pointer's current position. This helps visually ensure the connection is what you want before you click a second time and commit the action.
- Move the pointer to the target node and click on the second node to select it.
Hard and soft qualifiers for line and polygon feature types are used to indicate whether a distinct break in slope occurs on the surface at their location. A hard line is a distinct break in slope, while a soft line will be represented on the surface as a more gradual change in slope.