Apple Final Cut Pro 7 - Manual (Page 518)

Clips with both audio and video clip items create both kinds of tracks by default, unless
either the video or audio Source and Destination controls are disconnected.
Dragging a clip to the
unused space above
the highest track creates
a new video track.
After edit
When you drag a source clip to a track in the Timeline, all the clip’s items are linked. The
track you drag a clip to always receives a clip item, regardless of whether its Source and
Destination controls are connected. However, additional clip items are only placed on
tracks whose Source and Destination controls are connected.
518 Chapter 35 Drag-to-Timeline Editing