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Bouncing your leg rhythmically improves cognition if it
keeps you focused on your material instead of wishing you could
be elsewhere, moving in another way, and therefore distracting
you from your text. Your focus on your text and processing that
text contribute to improved cognition. Whatever helps — do it.
If you want to improve your cognition, start jumping rope.
Aim to improve the number of times you skip rope without
tripping the rope — and improve by at least one count every
day. Keep a chart. You will see that as your jumprope ability
increases, your cognitive ability will also increase. I don't
know if it is a 1:1 correlation, or even a causation. I do know
there is some correlation because I have seen it in every one
of my students.
Bouncing your leg is a small way to improve focus, which
improves cognition. But to really do it right, also have a jumprope
handy and use it during every break in studying or
working . . .
Matt Hastie adds an important link in his comment to
this answer, and I am taking the liberty of quoting from him,
here:
218
So why do kids with ADHD fidget and wiggle and run and jump
and bounce and scream and play so much? Kids with ADHD
are understimulated, which means that their thresholds are so
high, that the stimuli in their environment does not cause them
to release enough neurotransmitters to fit into all the necessary
receptor sites. Messages don't pass from one neuron to another as
easily as they do for those of us without ADHD. Their thresholds
are high. Kids with ADHD fidget and squirm in order to provide
extra stimulation, which translates into more keys fitting into
more locks, and they can pass messages efficiently. Ever studied
something intensely and then noticed that your leg was bouncing?
Same thing. You were bouncing your leg to stimulate yourself and
send a sufficient number of neurotransmitters into the synapse.
When kids have to stimulate themselves, it can be hard on everyone
around them, since this translates into bouncing off the walls.
Yvonne Kao adds these three points in her comments, excerpted
here:
1. Improved cognition associated with improved physical fitness
and motor coordination. There's lots of research showing this