The laser burns the first layer (L0) from the inside edge outward and then the second layer (L1)
from the outer edge back in toward the inner edge.
The laser has to refocus as it transitions from L0 to L1
Naturally, this takes additional time.
The Seamless option ensures that there will not be any annoying pause and is dependent on the dvd player.
Chances are you're ok as most players support this feature
Img provides a list of potential layer breaks and a preview option to view the cell where the layer break is positioned.
It's always best to choose an area where there is no fast action.
Personally, I've never had a "bad" layer break with Img.