drive.write(90) is neutral ?

School provide Mindsi robot rover 4x4 with below motor ESC sketch.

My questions in this sketch:

Why “drive.write(90)” is neutral ?

Why “drive.write(100)” is move forward ?

Why “drive.write(80 )” is move backward ?

Thanks


#include <MINDSi.h>
#include <Servo.h>

/***************************************************
/ Example provided by MINDS-i
/ Try checking out our arduino resource guide at
/ http://mindsieducation.com/programming-resources
/ Questions? Concerns? Bugs? email code@mymindsi.com
/
/ This example expects an ESC plugged into pin 4
/ It will also center a servo in pin 5 for convenience
/***************************************************/

Servo drive, steer;

void setup() {
//set a pin for the ESC/steering servo to use
drive.attach(4);
steer.attach(5);

//set the output for the ESC/servo
drive.write(90);
steer.write(90);

//delay 2 seconds for arming
delay(2000);
}

void loop() {
//drive forward one second
drive.write(100);
delay(1000);

//brake for 1 second
drive.write(45);
delay(1000);

//disengage the brakes
drive.write(90);
delay(50);

//drive backward one second
drive.write(80);
delay(1000);

//coast for 1 second
drive.write(90);
delay(1000);
}

(deleted)

I've never heard of a "Mindsi robot rover 4x4" but I guess the answers to your questions are because that's how many/most ESCs work, 90 = stop, numbers greater than 90 mean turn in one direction with varying speed, less than 90 is turn in the opposite direction with varying speeds.

Steve