Let's visit that bane of competition shooters, law enforcement qualification courses, and 'operator' training schools. That's right, the tactical reload.
What is it?
To put it country simple, a tactical reload is removing a magazine from your firearm (rifle or pistol) that you have fired some rounds from (but not all) and replacing it with a full magazine. You then keep the partially loaded magazine as a spare. This is different from an emergency reload where you are dropping an empty mag to the ground, then slamming home a full one in its place and charging the firearm.
Why use it?
In theory, the concept is that during a lull in a gunfight, such as while hiding behind cover/concealment and/or no active threats are engaging you, you pop out the mag you are working on and insert a fresh one. This allows you to resume the engagement with a fully loaded firearm.
The practice, this is at best a military or tactical team tactic that is practiced in fire-team sized groups, with 2-3 members returning fire or actively covering, while 1-2 members are reloading. In a personal defense scenario, private security, or solo law enforcement setting where you are alone, it is not as much of a viable option.
About the only exception to this may be for if you have a gun with a very short magazine capacity (say an LCP or LC380) and you get a few rounds off then want to top off to be ready for if the situation evolves from there.
(You have to practice swapping those mags around and seriously train for this before ever attempting to do so in the real world. In practice, its best to use an unloaded weapon and empty mags or mags filled with snap caps-- and then still keep the muzzle in a safe direction and your finger off the trigger)
It's all about timing
The main problem with the tactical reload is that it takes a person who is in a stressful, traumatic, life-threatening situation and expects them to amp down, remain calm, use fine motor skills, and fumble with two magazines in their off hand. With your blood pressure topping 200/120 and auditory exclusion blocking out the ringing in your ears, you can see the possible failure points.
If you feel like you have enough time to do this without a partner covering you, or a reasonable certainly that the gunfight is 99 percent over, then by all means practice it and train for it. Ideally this is something you should practice a few hundred times, from behind simulated cover after doing a bunch of jumping jacks, to become proficient. Without this type of muscle memory investment, you may wind up with only the round in the chamber when a gunfight resumes.
With this in mind, many instructors advocate speed reloading where you drop the partially used magazine free and index in a fresh one. This is a standard drill that you should have mastered already before moving on to training for tactical reloads. Following this premise, you can save seconds.
Emergency reloading is always faster than tactical reloading.
If you find yourself with extra time and can grab your dropped partial mag and pick it back up to store without taking your eyes off your threat, great. If not, move on and make your shots count.
Don't get me wrong, there are some isolated occasions where a tactical reload is the right thing to do. However, in most situations where you feel you should reload in a gunfight, a speed reload from cover with a fresh magazine replacing a mostly depleted one is the better, not to mention faster, bet. Another case of where the word tactical is more tacti-cool than practical.