While doing bench testing of motor and prop combinations what simple device would I use in the circuit to properly control the ESC for the full range of power from 0% to 100% ? I have seen a few videos where bench testing is demonstrated to measure the thrust and power usage of a brushless motor and propeller. Typically the control of the ESC is done with a very small device attached to the ESC? What is that device? (I find it hard to believe it as simple as a rheostat, or is it?)
To Control an ESC, or rather, give a control signal, one generally uses some kind of micro controller (arduino, esp32, Ti c2000, STM, countless others) with a ESC dependent control signal. The mcus themselves don't have to be large, they simply need to be able to provide a signal usually at 5v, which is trivial for them.
Some (most) of them require a PWM signal, such as the escons
The ESCON Module 24/2 is a small-sized, powerful 4-quadrant PWM OEM servo controller for the highly efficient control of permanent magnet-activated brushed DC motors and brushless EC motors up to approximately 48 Watts.
Others like the espos are controlled over a communcations protocol such as CAN.
The EPOS4 Compact 24/1.5 CAN is a dynamic, ready-to-connect positioning controller in a compact design. Designed for use with brushed DC motors with encoders and brushless EC motors (BLDC) with Hall sensors and encoders up to 36 W / 108 W.
If you're in the professional area, and in hobby areas
Things like the turnigy
Are also done with PWM,
Here is a post on using an esc with arduino.