Flashing is a term used to describe what happens when the folding is slightly off. It occurs if the front and back are the exact same size and there is any variation in the fold.
In this example, I colored the back of the paper orange to make the flashing jump out.
To protect from flashing we, make the back panel slightly smaller than the front.
If you look at the red line you can see a jump as it travels from front to back. This small cutback makes sure your cover is protected from any flashing.
4PAN1T Cutback allows for proper folding
On the digipaks, we use two types of cutbacks. One like the J100 above is that the inside spread is slightly smaller than the outside. This makes sure that there is no flashing on the bottom or right side of the cover when folded.
The second type of cutback we use helps the fold.
When you look at the digipak templates you will see that the area on the inside opposite the spine is cut back.
This cutback allows some room for the spines to fold. If there was no cutback on the spine the board would bunch when folded causing buckling and bulging.