I have covered multitasking before, but I don't believe I've covered it in depth enough. Recap first, the human brain cannot truly multitask, similar to the computer. As long as there's only ONE physical core, true multitasking is not really possible.
Lately, because my to-do list has been getting longer, I've ended up multitasking a quite a bit while on the computer. I had: an internet tab on facebook, one on a forum searching for programs, one on a site searching for japanese games, one on a forum looking at posts on learning the japanese language, one looking at a site for learning japanese recommended by the thread, 3 for searching for trackers for a slow moving torrent (naturally my torrent client's window was also open for me to monitor it), all this while switching back and forth between listening to Houkago Tea Time and an Anime/j-rock radio station.
I naturally didn't listen much to the music although it did provide enjoyment. As for all those windows, I ended switching back and forth between tasks. Now studies have shown that multitasking too much can cause problems with attention spans and concentration levels. The research on mental effects of multitasking may be little, but it certainly sounds logical and thus plausible. Nevertheless, I believe for this particular case, it's probably better.
You see, I'm doing a search (a few in fact) and basically on the lookout for anything useful. This means that only after a length of time of not finding anything useful or interesting will I terminate the task. Now this can be dangerous. Much like early computers doing cooperative multitasking, if a program wasn't programmed to pause so that others can run, it could end up hogging the CPU, and thus the system will appear to hang. Same thing here. I've done searches which keep yielding interesting results and end up branching as well into their own searches. I'm an inquisitive person by nature so this situation is very common. As we all have limited time, this constant hit of results cause me to get stuck on one task, or more, but generated from the same parent task, and thus not complete others. You could call this a problem of focus, but if I were to breakoff the endless task and be so-called more "focused" then I will end up missing out on potential useful information. (Remember my search isn't for something specific in the first place.) Writing down what I want to do next is troublesome and time consuming, not to mention that it is only temporary, and it's easier to recall what you wanted to do from the page itself than the text.
The easiest would be to just leave the tab there and switch to the next task while it loads. This is also what a computer does. Preemptive multitasking. A scheduler built in interrupts task which are, like, waiting for something to happen, e.g. Input from user, new information to be fetched from harddisk, etc. In this case, I interrupt the task while waiting for it to load (in the case of viewing webpages), or complete its job (in the case of searches), and move on to the next task in order. In the end, I accomplish a balance of all tasks and use up all time available. This would be opposed to either accomplishing only one major task, but in depth (as when I get caught on to one task alone), or accomplishing all tasks without depth but ending early and thus not maximising time (as when I take the so called "focused" approach). Both are not as desirable.
The disadvantage occurs however, if task switching takes a long time. This applies both physically and mentally. A switch from website tab to tab is almost instant, and if you're doing similar tasks like searching, then it is easy to switch and remember what you're doing. A switch from a full screen app to another full screen app may be rather unproductive even if they are two instances of the same app (which doesn't tax your mind in this switch) simply because the com can't switch that fast. Likewise, if you switch windows from email (casual) to say, a school essay, and then to a sports forum discussion, you will find it mentally challenging to switch mindsets. If you don't get this, think about how fast you can write two notes sincerely with empathy: one congratulating someone for an incredible achievement, and another consoling another about a great tragedy. Your computer of course would not find it difficult to switch between those windows. The bottleneck thus, is you in this case. Basically, in multitasking, it just depends on how fast you can switch tasks. The faster it is, the better your performance will be.
On a related note, this brings up the ideal. If you're performing a series of related tasks (not just similar), and there is a need for multitasking, then your performance will be pretty much improved. This form is similar to Intel's hyperthreading. The reason is because the tasks are related, your mind has no problem switching, and you can perform the main task faster. A good example I suppose would be using different search engines to search the same thing for purpose of better coverage, like pictures. Wikipedia describes Intel's hyperthreading as several tasks accessing the same bits of memory. This means no time is needed to fetch more memory from other places to enable switching of tasks.
Be careful though, multitasking only works on certain tasks well. Much like the new series of processors, Intel's i3, i5, and i7, some single track tasks will perform much better without multitasking (due to turboboost if you know what I'm talking about). For instance, I can't listen to music while typing this post. It'll be too distracting, and I'll not be able to organise my thoughts properly.