For whatever reason, you had to pull a guy out of his cell. Used to be for any disciplinary reason: he was tearing things up or making so much noise the other inmates couldn't sleep. Nowadays, we would just let them vent. In order to be a Threat, a person must have Intent, Means and Opportunity to hurt you. Going into the cell would give Opportunity, the third element. We would be responsible for making him a danger. So only self-harm (for which there is always Opportunity) really justifies entering.
So the tactical problem, back in the day, was that you would have to go into a concrete cell through a standard (except for the steel reinforcing-- standard sized) doorway. On an inmate who can see you clearly, is completely prepared for you, may have weapons, sometimes made armor out of blankets and occasionally soaped the floor. Commonly in a boxing stance just inside the door.
Even in Corrections, at least in our system, this is an obsolete problem. But before the Team and pinning shields, when no one was allowed to carry OC (pepper spray) or Tasers, it both common and dangerous. An unarmed or semi-unarmed classic 'funnel of death' problem. Only one person could go through the door at a time...
It's obsolete even for Corrections. Enforcement always had force options available, so this never was an issue. For citizens, there is no intelligent reason to close the distance on someone who is prepared to fight you. That makes it a remarkably useless technique, but technique isn't the point. It's all the nuances that made it work that apply to everything.
What follows is the first actual technique I came up with as a DT instructor. Sgt. Gatzke presented the problem.
It's simple, really. Hands up, non-threatening, you approach while talking. As you get to the Critical Distance Line you drop-step hard and fast, forward and very slightly off-line to his lead side, pass/parry the lead hand and voila, you are at his flank or behind him, out of the danger zone and with a plethora of options.
Approaching, hands up and non-threatening. Hands are in some version of the Fence (as described by Al Peasland who learned it from Geoff Thompson). Lots of people screw this up. If you look angry or tense, like you are ready to fight, not only will the threat have a better chance of clocking you when you make your move, the tension will slow you down. You must carry yourself like you believe talking will defuse the situation.
The second killer little detail is that you must walk unnaturally. Humans are direct-register animals, like cats. When the left foot goes forward, the right hand goes forward. In this instance, your right side moves together. Done fluidly enough, I've never had a threat notice.
As you get to the critical distance line. You must be able to read exactly when the Threat can reach you. Most of the ones I've used this on seemed to have boxing backgrounds and I needed to know exactly when I was coming in range for the jab. You had to be able to read when any shift in feet or center of gravity altered the critical distance line or loaded a different limb.
You must time it so that you will cross the Critical Distance Line with the mirror side. If the Threat is in a left lead, you must cross the CDL with your right hand and foot. That makes the distance for your drop-step and parry as short as possible. Short distance makes for better speed.
And you must be smooth. If you try to adjust your footing on the way in, the walk will be unnatural and the Threat will know something is up. You can't stutter-step or skip to get to the CDL with the right foot forward.
While talking. Sort of. You also want to judge the distance and speed of approach such that when you cross the CDL, the Threat is talking. Not critical, I've made this work on a number of Threats who were stone silent and watchful. But if you can get him talking there will be an additional delay, just a fraction of a second, before he figures out what is going on. For that matter, look at everything going on in this technique and see how much of it is about shaving fractions of seconds. Talking guys have slower reflexes.
Drop-step. A good drop-step is an incredible power and speed multiplier. Unless you can throw yourself to your feet from a push-up one handed, you fall harder than your arms can hit. Because gravity is always on, there is no delay and no natural telegraph. Unless you screw it up with hesitation movements or weird breathing or something, you will be moving before the threat sees, and probably finished moving before he can react.
The drop step is slightly off line. You don't want to fall into him or into his punch. You fall (and can add a fencer's lunge for more distance but DO NOT sacrifice the drop step and surprise by loading the lunge) to just outside his lead foot. The angle takes you off line of jabs and crosses. The drop tends to protect you from high hooks and roundhouse punches. Low hook you would have seen coming from his hand position at the start.
The drop step has to be practiced, by the way, and I've seen some very odd things called drop steps.
Pass/parry his lead hand. I learned it as the mirror block decades ago. Hard to describe in print. Basically, both open hands make part of a circle. The lead (right hand for this example) has the palm to the left and comes from belly button to ear, ending with the back of the hand right near the left ear. The left hand, palm facing right, follows from the other rim of the circular action.
You don't block. Ideally, your drop-step was just enough out of line that the punch, if it happens, missed with no contact. The mirror block is just a form of insurance, with the added benefit that that the left hand will come in contact with the leverage point at the distal end of the humerus. Especially if you have practiced forearm rolls, you will have the leverage and power to completely turn even a much larger man.
Oh, and trained karate guys tend to screw this up because they point their fingers up like a shuto block, which costs them four inches compared to the far more comfortable technique known as a handshake.
This all puts you in his dead zone, that special place on the rear flank where it is very difficult for him to apply power. You have force options ranging from a simple off-balancing push (or an ear-splitting scream I guess, if you want a very low force option) through spine controls, joint locks, damaging and crippling strikes all the way up to lethal force.
Again, don't nut up on the technique. It's obsolete anyway. Just as in breaking two fighters apart there is a lot more to think about than just the technique. How people walk naturally and how to exploit that. The need to train your eye for reading range and balance shifts. Application of gravity for speed. How communication affects reaction time. Lots of stuff.