Assuming you're using a brush that is completely curve-based (topology brush or zremesherguides), you can just draw a second curve that bisects the first one. A point will be created at the spot where the curves intersect, which will essentially split the curves into multiple segments. You can clean up curve segments by drawing a new curve over a segment while holding alt. Using these two methods, you should be able to shorten curves, add gaps to curves, etc.
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