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The following image shows how this works. The top panel is the IGB main display window shows the normal, non-spliced view of a region containing overlapping genes from a cow: GBN4, which contains an enormous five prime intron that completely encompasses another gene, ZNF639, which encodes a zinc finger protein. The Sliced View panel on the bottom shows these same two genes, but with the intronic sequences reduced to 100bp. The places where the slices were made appear as black hash marks below the axis in the Sliced View. Note how the sliced view makes it easy to see both gene structures simultaneously, whereas in the main view ZNF639 is so small it is impossible to even see the number of exons it contains.
You can enter the size you want the introns to be set at by changing the number in the Slice Buffer (default is 100). By unchecking the Slice by Selection box, you can move the main view to another region of interest, and click on other features without changing the sliced view.
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