it isn't really beneficial for a parent, it is forced :D
I don't think those examples are true multitasking though as the processes are habituated so no thought is necessary. The drummer doesn't need to think about the bass pedal after a thousand hours training, the hunter doesn't have to think about whether they can use their weapon or not. The problem is when two trains of thought have to travel together in one mind simultaneously, it is like crossing your eyes, not an improvement for vision.
That is actually very well know in professional E-Sports. If you need to be 1/5 of a second faster then your enemy, you have to prepare yourself with extreme idle APM (Action per Minute) and also with the mindset of acting and expecting moves to come, even if 99% of them are in vain.
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You can see him getting slower, as soon as something really is happing. The fast clicking and moving is just constant preparation.
After all, I as a person do have rhythm. If actions or thoughts break that or don't fit in, then I slow down - getting very very slow.