So there's this wonderful little icon that when it appears, makes comping my various tracks soooo much easier. 
 
It's this one: 
 
Specifically it's designed to add crossfades between two adjacent clips, which is awesome in itself, but the most handy thing it does is move both adjacent clips in unison so I can set the exact transition point I want. 
 
The only problem I have is that it seems to be erratic in when it will actually show up. Many times it will show up when two clips are overlapping, when it does this it inserts a split where I click and allows me to drag & adjust as described above...very cool! Other times it just doesn't appear. It often won't appear as well even if two clips are perfectly adjacent. And it's frustrating when I'm moving my little smart tool around in vain trying to find that perfect hotspot for it to show.
 
What conditions must be met in order for this to appear in the smart tool?, Honestly, I would love if there was a consistent way it could be implemented, as it takes what is normally a tedious 4 step process (slip edit both ends of the clip, then add cross fades to both ends) into a single mouse stroke.

After debating the worth of comping on this forum recently, someone else mentioned the crossfade icon. I've never seen it, and couldn't get it to show up at all.

The icon only shows up when:

1. You are using, and showing, layers within the same track.  
2. When the clips are *perfectly* aligned.  I have had many instances where I thought they were but zooming all the way in shows a slight offset. I use snap to marks (disengage smart grid as that can throw it off) a lot to align clips for comping.  Alternatively you can drag one clip (the entire clip, not just adjusting the boundaries) over another and Sonar will then cut the clips to create a perfect alignment. Then use "move audio within the clip" feature (alt+shift plus mouse drag) as needed to move the audio back into position.
3. When conditions 1 and 2 are met, you only see it if you hover over a specific part of the comping area.  I am not at my studio now, but as I recall you need to hover over the lower half of the clip to the right.

It is a bit touchy, but once you figure out all the rules it works well.

I know that it's shown up in times when two clips overlap significantly. And I have cut/pasted clips using snap settings so they should be perfectly aligned and yet it doesn't appear. This is actually when it would be most handy. So it seems that there is another condition in which it shows up.
 
Honestly, I just wish there was a quick, reproducible method to select two clips and slip edit them simultaneously, without having to create the perfect conditions and hover over the exact location. This makes workflow 100x easier, especially when splicing together different takes in a track.