Inside the Select Tool, double-clicking with the left mouse button invokes various shortcut actions that are both context-sensitive and modal. If you are in Face Group Color mode, then double-clicking will append the entire face group that is under the cursor to the selection. Doing this action with the Ctrl/Cmd modifier key held down will deselect the face group.
In Vertex Color mode, this action will do a sort of selection flood-fill. All triangles connected to the triangle under the cursor will be selected, but the flood-fill will stop at triangles that were already selected. So, using this action you can paint a selection boundary and then quickly select the interior.
Boundary Selection
Regardless of color mode, if you double-click on a triangle that is adjacent to a boundary (highlighted blue edge loop), the selection will grow to include all triangles with a vertex connected to the boundary loop, as shown below. This is an efficient way to select hole boundaries, which is particularly useful when cleaning up scans and "problem" meshes.
Select Intersecting
Double-clicking is overloaded with one more interaction, which allows you to use other objects in the scene as selection tools. If you double-click on some other object in the scene, all the faces fully contained within that object will be appended to the current selection. Note that since only fully contained faces are selected, you won't actually be able to see any change until you hide the object you double-clicked. As with other selection tools, the Ctrl/Cmd modifier key will invert this operation, removing the contained faces from the selection.