To insert a clip to the timeline using the Insert button:

Depending where the positioner is placed, the clip is inserted, and the overall length of the timeline sequence is changed. The following example shows a New Clip inserted at the transition between Clip A and Clip B. Everything after the transition is moved ahead in order to make room for New Clip.

This example demonstrates the result of inserting New Clip at a frame in the middle of Clip B. Every frame after the edit point is moved ahead to accommodate New Clip.

To insert a clip to the timeline gesturally:
As you hover over the timeline with your clip, you see a phantom visual guide as to the space on the track or tracks your clip will take when dropped.