When we are multi-tasking, we aren't really doing two things at once. We are really working on one activity, pausing it while keeping the current status in working memory, and then switching to the second activity. And then the same thing when we switch back.
So this depends on two abilities. You have to have good working memory to keep all of the first activity stored while you are working on the second. Otherwise, you have to review what you have forgotten every time you switch. This is no longer multi-tasking.
The second ability you need is attention switching. The attention blink is the delay that occurs when you switch from one activity to the other. Your brain can't do it instantly. There is some amount of time that your brain needs to refocus on the second activity. This is just lost time. It makes multi-tasking, even when you have good working memory, inefficient.
So people with big working memory capacity and quick attention switching ability better at multi-tasking than people who have less.
Interestingly, a new study in the journal Neuropsychology reports that psychopaths have great attention switching ability. Apparently, their brains are perfectly happy to put aside one activity and move to the next one. I'm not sure why, but I thought this would be a fun fact to add.
The study I want to talk about in a little more detail is the effect of emotion. A new study by Helen Tibboel in Cognition and Emotion found that when the activity we are switching to is an emotional/arousing activity, we can switch faster. This is related to the fight or flight instinct. It also helps switch from texting to braking when someone cuts in front of you on the highway (assuming you notice it). Our brains have somehow learned that emotional/arousing things are very important and worth dropping the previous task, even if it means forgetting more of it. So it will take longer to go back to the original activity because we will have to review more of it to get back to where we left it.
This has good points and bad points. It means in the workplace, when there is a safety issue (which is usually high arousal) we can switch to it quickly and protect ourselves. But it means that every time this happens, it takes longer to get back to work, so productivity takes a big hit. It is well worth it for companies to invest in eliminating safety risks, even if no one is getting injured, just because of this impact on productivity.