Lassoing is a very fast method of selecting segments in both Source-Record Mode as well as Effects Mode. It is also a great way of entering Trim Mode. Beginning while in Source-Record Mode, here’s how:
  
Lassoing Method One:
Click OUTSIDE the Timeline (in the Timeline Window,but not within the Timeline itself) and continue to hold down the mouse button. It is generally easiest to start above the timeline, but it works starting from either above or below the timeline.

 
Lassoing Method Two (Lassoing Inside The Timeline):
Now the question arises: What if you want to lasso a segment/s or a transition/s that is/are in the middle of all the tracks in the timeline. In these cases, hold down the CONTROL¹ key (Mac) or the ALT key (Windows). When you do this, notice that a small, black box appears at the tip of your cursor. Now, while continuing to press down the Control¹ key or Alt key, you can lasso right in the middle of your timeline!
¹ NOTE: Starting in version 5, Mac users must use the OPTION key to lasso inside the timeline.

 
Lassoing Uses
If you lasso around ONLY A TRANSITION (the black line between two segments), and then release the mouse button, you will automatically enter Trim Mode (fast eh!). And you can lasso multiple transitions across both video and audio tracks.
 
If you lasso over one or more ENTIRE SEGMENTS while moving your mouse FROM LEFT TO RIGHT, you will go into Segment Mode. Specifically, you will enter the type of Segment Mode you used last.
 
If you lasso over ONE SEGMENT in the direction of RIGHT TO LEFT, you will be put into SLIP MODE. If you hold down the OPTION² or ALT² key while lassoing over one segment from RIGHT TO LEFT, you will be put into SLIDE MODE. Note: you may also Slide multiple segments.
² NOTE: Starting in version 5, users must use OPTION + SHIFT or ALT + SHIFT.