Ah, the perennial "tactical reload" dilemma.
This wasn't such a topic for discussion back when we carried revolvers. Nor is it much of a critical training topic nowadays for folks who carry revolvers as either secondary weapons (on-duty) or off-duty.
Yes, when we transitioned from revolvers to duty pistols the training common to that time came to involve "tac reloads", and some of us who have been doing them since that time have developed the ability to do them without having to stop and think about it. By now it's more of a habit, than anything. I've watched some of our older instructors do a "tac reload" after firing several shots, moving to a different shooting position (like to cover, or to another barricade position), do one without even having realized they'd done it until someone asked about it.
Yes, it's done with retention, and involves bringing the new mag to the weapon, replacing the existing, partially depleted, mag with the fresh one very quickly.
The rub usually comes when deciding what to teach to new LE shooters from time to time, and
when to emphasize the purported practicality and utility of the "tac reload" to newer shooters.
The idea of a "lull" in an active shooting incident looks good on TV and in some movies, and may be something designed into some training or competitive venues, but it's not an easy one to find illustrated in LE shootings, let alone non-LE private citizen shooting incidents. Or, as the guy currently running the FTU at my former agency once asked an earnest, less experienced firearms instructor who wanted to have more emphasis on "tac reloads" in some training, find him 5 examples where it happened and was useful during LE shootings in the last 50 years. Then, show him where it actually was demonstrated to have been a critical advantage during those shootings, versus reloading from slide-lock.
We still teach it, but its emphasis is not very high on the list of manipulative and shooting skills considered important or critical for most beginning and average skilled shooters to learn to master. It's more of an intermediate/advanced skill taught at some point, and the relative advantages/disadvantages are usually discussed whenever it's being required as part of training or general quals, leaving it up to the shooters to decide, off the range (on the 'street'), when/if it might be appropriate for them to use in an actual situation.
Another thing to consider is whether an "over emphasis" on retaining a mag for a "tac reload" may confuse someone, and therefore hinder them, in performing a manipulation to resolve a mag-related stoppage & functioning problem
where the existing mag is thought to be the cause of the problem.
If we're teaching strip/dropping of the primary mag, for replacement with another mag to resolve a mag-related issue, is putting a priority on retaining the primary mag in a "tac reload" going to cause confusion, or a skill mix-up, for the suddenly stressed shooter having to now resolve a mag-related stoppage problem in the midst of a shooting incident?
Personally, I'd rather ingrain a smooth response for addressing a possible mag-related problem, and that generally means stripping/dumping the suspected problem mag so a fresh one can be inserted as quickly as humanly possible, under stress. An "emergency" reload, if you will. Just as much of an emergency as a suddenly dry weapon. The partially depleted mag, suspected of being involved in causing the stoppage, can be discarded and become part of the evidence at the scene, and be carefully examined at a later time during the investigation.
Nowadays I see the idea of a "tac reload", meaning with retention, as being more of an advanced skill, and probably not one that's going to be employed in very many situations where a LE shooter has encountered a "lull" in their dynamic, chaotic, rapidly evolving and highly stressful shooting incident.
Those of us who were required to invest so many years and countless range hours to learn, practice and ingrain the skill? Well, it's nice to be able to do one without having to stop and give it a lot of thought, and be able to demonstrate it to newer shooters as a variation on a manipulative skill that might, maybe, become useful some day, I suppose. Perhaps we're just unwitting 'victims' of the earlier training of our time.
By that, I mean those of us who once carried service revolvers, and who didn't really emphasize a specific technique to "top off" our partially spent revolvers using a dedicated "tac reload" ... although there were still a lot of bullet loops and dump pouches (for loose rounds) seen on gun belts back in those days, even with speedloaders pretty much being ubiquitous. I do remember some of the guys who wore Buscadero gun belts having not only bullet loops, but a separate single or double bullet loop on the front of their belt, ahead of their holster (meaning with the bulk of the bullet loops located to the "rear" of their holster, on the back of their belts). Those were usually considered to be placed on the gun belts so they were more easily accessible, if the shooter only fired 1 or 2 fired rounds and wanted to replace them.
Just some thoughts.