'Split back' labels are as they sound - a small slit is cut into the backing liner of a sheet label product.
When is this important?
Some customers will order uncut sheets of adhesive material for custom printing. When there isn't a kiss cut present, separating the label from the backing liner can be difficult. Applying a slit back will help the end user to peel the top adhesive layer away - similar to the back of an adhesive plaster you may find in a first aid box.
Another peculiar term for this technique is 'crack back.'