1. Decent amount of taperand diameter. This means the "choke point", or smallest cross section is bigger than the TB, and short, which enables a higher air speed without the associated pumping loss.
2. "Cobra head" style elbow. This acts as a mini plenum, giving the TB a sort of reservoir, instead of just a regular 90° pipe bend. This enhances the air speed profile around the perimeter of the TB, resulting in a higher average air speed for a given max. It is the maximum air speed that limits how much air can be moved without "choking". The more even your "velocity profile" is, the more air you can flow without crossing the velocity threshold. The cobra head elbow allows the TB to pull from all 360°.
You gotta keep in mind that moving air faster gets more in the engine, but it takes energy to move it. There is a certain air speed that makes the most power. Slower, and you loose power from lack of airflow. Higher, and all the power the extra air makes is simply wasted on accelerating the air, itself. It's all a matter of finding the majical spot in the middle.
Hope this helps.