Googled, but still do not understand what "interstitial effect" means. Can someone please explain?
The root meaning of an interstice is that of a narrow but possibly deep gap or fissure that opens in a solid material, often as the result of the action of antagonist external mechanical forces.
The interstitial effect refers to the phenomenon of some fluid filling up these gaps.
Here are a few examples purposely taken from very different domains.
In geology. For instance a fault in the Earth crust can be filled by a surge of basalt from the underlying magma. This will be named "interstitial basalt". This is also the mechanism at the root of the nice calcite veins in marble or the inclusion of flint stone in chalk beds.
In medicine. If your bladder lining is leaky, there will be fluid or alien material filing up the interstices and you will be subject to the condition named interstitial cystitis.
In economy the substrate of small businesses is the nursery for future large companies provided they can identify and address high potential product niches. Here again there is an interstitial effect if you consider that these small businesses are more flexible and quicker to adapt to new market demands. The interstices are the niches opening between the mainstream traditional markets already dominated by large companies. In turn, large Companies neutralise theses start ups by purchasing them (see the buying records of Cisco, Google or Oracle for plenty of examples of this phenomenon).
In ecology, the interstitial fauna is the fauna that live in the nooks and crannies of a beach or some other open space.
In the web industry, interstitial pages are ad pages inserted in the course of the web navigation of site visitors.
It seems what you are looking for is a transition from one thing to another. For example something between ads
Here is an animation made to be an interstitial between clips
Title: Solar Explosion
Resolution: 720 x 486
High impact title or logo animation which can be used to open or close your show as well as an interstitial transition between segments. There are 2 projects included, one which requires no plugins as the effects are pre-rendered for you..
there are some examples of the usage of that word here and here's a good one that can help you understand the meaning:
"I can't predict what will happen with interstitial art, but it will continue to evolve (as I think it has for quite some time now, under the guise of other nomenclatures) and if "interstitial" is not the term that defines this particular kind of art-making in the future, that's fine." — EXCLUSIVE INTERVIEW: Delia Sherman and Christopher Barzak on Interfictions 2
Interstitial also refers to the place between floors of a building (above the ceiling of a lower story and the floor of the story immediately above).