You are right. This is because when you exhale, it calms the amygdala, which manages fear. The pattern of breathing in meditation over time causes a cumulative effect, and you end up with less stress, and less inhibition. At the same time, if doing a meditation where you direct focus, you are priming parts of your brain involved in focus and attention. Like I said before, motivation is an equilibrium involving inhibition (potentially caused by fear). Meditation is great for motivation.
I wouldn't go as far as to say most cases of procrastination would be fixed by meditation, though. Procrastination can have multiple causes. I'd say a failure to have clearly thought something through or unconscious psychological resistance would be more likely factors in procrastination, and meditation would not remove that stress as readily as logic or psychotherapy would, respectively.
(Just reordering my own thoughts here; I'm no expert.)
Cortices.
Very interesting comment! I find it slightly paradoxical that I associate Buddhist meditation with a passive attitude of just accepting the present and suppressing earthly desire, yet according to what you say, that activity actually improves one's ability to persist in the active pursuit of one's desires. Not a true contradiction, of course: both are instances of suppressing immediate desires in favor of more significant, delayed ones.
Besides any long term neurological benefits, I read once or twice that meditation and relaxation have an immediate effect of helping relieve the stress that lies at the root of most cases of procrastination.