The principle behind emergency reloads is to conduct them fast and get the gun back into the fight. If the magazine is not retained in a mag pouch, you may find yourself able to reload the rifle in under 3 seconds. But this is of dubious value in my experience.
Let me just say that 3 seconds is a long time in a gun fight. You can get shot more than ten times in less than 3 seconds, with all the rounds hitting beautifully in your center of mass. For this reason, I recommend that you put priority on getting out of the line of fire.
When you are engaged with a threat, 3 seconds is plenty of time to move from one point of cover to the next. Some say that taking your eyes off the threat for a reload isn't a big deal since you can't do anything about the threat while the gun is down. This is far from true in the real world. Just knowing that your threat is moving can have an enormous impact on what you do when you get your rifle reloaded. Maybe you will even see them pop around cover to engage you. This may save your life and allow you time to get your head down or at least react in a way that doesn't leave you vulnerable.
Doing the reload right seems to have taken a back seat to doing it fast. funny thing is that I have seen this go terribly wrong when real stress is added. People tend to try to go so fast that they end up messing up the process or skipping steps, resulting in them having to take more time to correct their mistake.
My recommendation is to conduct your reloads in a way that will work every time, no matter the variables involved. Pulling out the magazine instead of trusting the magazine to drop free is a huge one for me. Also i recommend using the charging handle to get the rifle loaded so you don't have to change your routine if the rifle all of a sudden decides to not stay locked open after the last round fired. I have encountered this problem before and it is not pretty or fun.
No matter what method you use for speed reloads in an emergency, you need to remember that you are fighting a threat that will shoot back. There are a great many variables that go into it, and it is on you to be mindful of them. Train hard, and train often.