Ok here it is in a nut shell. First lets grab some "what if" numbers and keep it simple.
You have a cam with a 1" intake and 1" exhaust, what this means is when the lobe of the cam is at its highest point it is 1" above its neutral line. (of course this is much bigger than an actual cam but makes for easy math
Now the cam has a hydraulic lifter that rides on the cam, this lifter then pushes a Rod. This Rod is seated (ie mounted) into one end of the "Rocker". The rocker is mounted to a pedestal and works like a Teeter Toter or See Saw (playground toy). The valve is held closed by a very strong spring and retainer, the valve is seated (ie mounted) to the other end of the Rocker (there are two ends to the rocker, one end has the valve the other end has the push rod).
So when the cam rotates it pushes up the Hydraulic lifter and the push rod, This causes the Rocker to push down on the spring making the valve open.
Which takes us to Ratio'*.
When the cam lifts 1" and the rocker ratio 1.6 to 1 (stock size) the valve opens 1.6"
When the cam lifts 1" and the rocker ratio 1.9 to 1 (high ratio size) the valve opens 1.9"
The bigger the opening the more the flow. But dont look at the hole size as being the only opening?
Think of it this way, take a beer can a poke a hole in the side, now put your finger over the hole but not against the can, get it really close within a few thousands of an inch.
Beer will still flow out but not as fast as when it was 'wide open'.