When it comes to shotguns, they are odd in that we can load a single round through the ejection port if we have managed to run the gun completely dry. Emergency loading, regardless of platform, is in my opinion an important skill to have. It falls into the category of tasks that are statistically a low probability, but have very high consequence if we cannot accomplish the task efficiently.
There a couple different approaches to accomplishing this with the shotgun. Assuming the use of a side saddle, one technique is to come over the top of the receiver, drop the shell into the ejection port, get back to shooting. This is generally assumed to be the most efficient method, but with the addition of red dots to shotguns or with some semi-auto shotguns, there can be complications.
The other variation is to approach the ejection port from underneath the receiver. This has the benefit of avoiding any optics on the gun, and positions the hand well for either running the action closed on a pump gun or activating the bolt release on semi-auto guns.
So the next questions that comes up is which is faster. All things being equal, faster is better in terms of emergency reloads.
But are all things really equal? I suppose this is up for debate. In my opinion, over the top is easier, if you run the shells in the side saddle with the brass up, as I do. However, it also means I have two distinct methods of loading, one for emergency loads and one for topping off the magazine tube. The argument for the coming under the receiver is that the technique is very similar to the technique used for topping off the magazine tube, creating continuity.
Let me know how you run your shotgun in the comments.