Maybe I can clarify the vacuum/ pressure discussion.
Everyone of us is living our lives being subjected to atmospheric pressure. Here on earth that pressure is approx. 14.7 pounds per square inch at sea level. It varies with the weather (barometric pressure) and will be lower when standing on top of a mountain, more at the bottom of a canyon that is below sea level.
That pressure you are subjected to comes from the column of air that is above you from where you are standing to outer space. That column has weight and you have some square inches of surface area, divide the weight in pounds by the square inches and you get pounds per square inch or psi.
For convenience stake pressure above atmospheric is referred to as positive pressure and below atmospheric as vacuum.
Someone mentioned the ideal gas law. In this law with all other things remaining constant pressure varies inversely with volume. That is volume goes up pressure goes down. This relates to your engine in that you have a volume of air above the piston at atmospheric pressure. When the piston starts down it's stroke that volume above the piston gets larger and the pressure goes down. The air outside the cylinder is still at atmospheric so when the intake valve opens the higher pressure air moves to the lower pressure air to equalize.