^^^ Yes you are rite in a basic answer however there is other reasons for this restrictor.

A lot of designs are around but they all go buy a basic princible of how the air enters the base of the carb this princible is for all forms of motor sport cars trucks bikes and rc cars

longer stacks create more torque compared to a similar designed shorter stack

were as a shorter stack of the same design can create more rpm

however there is a point were you cancel it all out by going to long or too short

its manily used as a tuning tool

the theroie (spelling) is the air going through the slide potion of the carb is a lot denser with both designs however the longer restricter slows it down there for creating more torque but still making it dense,were as the shorter stack it goes through a lot faster making it give the engine more rpm with less torque.

i tried my best to dumb it down if you wont more of a detailed explanation just ask also you need to take into account the ve of the motor as well for it to work or just try a few things and see what happens