It's actually a lot simpler than you might think to do this. To slip a layer, you don't even need to go into footage - just drag the layer timebar in the timeline to start where you want. For more precision, go to the frame you want and hold down shift while dragging the timebar, you can snap the layer's start point to the current frame this way.
This is how clip timing works in combustion: footage contains the trimming settings for a clip, i.e. the in/out points of a clip. This will be used everywhere that footage is referenced. Once that clip is referenced as a layer, you can further trim the in/out points from the already defined ones by dragging the end points of the layer, OR you can just slide the layer around to control where those already defined in/out points on the clip start in relation to the composite's duration.
Hope that helps,
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Greg Niles
combustion Product Specialist
Discreet