Lock Tangents lets you manipulate handles of multiple vertices simultaneously. 
 
When Lock Tangents is active, dragging a tangent handle affects the handles of all highlighted keys. When Lock Tangents is inactive, dragging a tangent handle affects only that handle's key. 
Procedures
To drag discontinuous tangent handles:
- Highlight one or more keys in the Curve Editor. Hold down the Ctrl key to add to the selection set. 
 
- Click 
 (Lock Tangents) to set one of the following modes: 
- 
Off: Dragging a tangent handle affects only the single key displaying the handle. 
 
- 
On: Dragging a tangent handle affects both handles of all selected keys.