How is attention involved with the performance drop associated with multi-tasking?

What influences on attention cause the productivity of a multi-tasking person to be low?

An example of multi-tasking is that a person is working on his assignment while trying to memorize unfamilar Spanish words.

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1. listen to someone and read something at the same time
2. listen to someone and walk
3. walk and read something at the same time
$\#1$ is generally harder than $\#2$, partly because both of its tasks involve linguistic processing, whereas walking does not.
$\#2$ might be a somewhat unfair example at first glance, because walking is also more self-contained and automatic on flat ground. On uneven ground, however, $\#3$ would be much harder than $\#2$, because both walking and reading require visual processing in such circumstances. This point implicates automaticity and difficulty as (probably related) influential factors as well. I'm sure there are others, and I hope others will help fill in other ideas.