When the throttle plate is closed, there is a big pressure difference between manifold vacuum and atmospheric pressure. When you snap the throttle, the pressure, RAPIDLY, equalizes .
( 1 ) This RAPID in-rush of air is the first rise. ( 2 ) Now, with the pressure equalized, the in-rush of air, drastically, slows down. ( 3 ) Then, as the RPM's rise, air flow rises, again.
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The following user(s) said Thank You: Noah, Andy.MacFadyen, Monde