Arduino Code for Pulse Skip Modulation (PSM)
Just adding to chrisl's answer since I bought this module, too.

There is a simple example on the manufacturers website of this module which does not seem to implement PSM the obvious way:


/*
Connect RobotDyn AC Light Dimmer as follows:
Dimmer pin  --  Arduino pin
    VCC     --    5V
    GND     --    GND
    Z-C     --    D2
    PWM     --    D3

Glow sketch for AC Voltage dimmer with Zero cross detection
Based on the sketch by Charith Fernanado 
Adapted by RobotDyn: http://www.robotdyn.com
License: Creative Commons Attribution Share-Alike 3.0 License.
Attach the Zero cross pin of the module to Arduino External Interrupt pin
Select the correct Interrupt # from the below table (the Pin numbers are digital pins, NOT physical pins: digital pin 2 [INT0]=physical pin 4 and digital pin 3 [INT1]= physical pin 5)

Pin    |  Interrrupt # | Arduino Platform
---------------------------------------
2      |  0            |  All
3      |  1            |  All
18     |  5            |  Arduino Mega Only
19     |  4            |  Arduino Mega Only
20     |  3            |  Arduino Mega Only
21     |  2            |  Arduino Mega Only

In the program pin 2 is chosen
*/
int AC_LOAD = 3;    // Output to Opto Triac pin
int dimming = 128;  // Dimming level (0-128)  0 = ON, 128 = OFF

void setup()
{
  pinMode(AC_LOAD, OUTPUT);// Set AC Load pin as output
  attachInterrupt(0, zero_crosss_int, RISING);  // Choose the zero cross interrupt # from the table above
}

// the interrupt function must take no parameters and return nothing
void zero_crosss_int()  // function to be fired at the zero crossing to dim the light
{
  // Firing angle calculation : 1 full 50Hz wave =1/50=20ms 
  // Every zerocrossing thus: (50Hz)-&gt; 10ms (1/2 Cycle) For 60Hz =&gt; 8.33ms (10.000/120)
  // 10ms=10000us
  // (10000us - 10us) / 128 = 75 (Approx) For 60Hz =&gt;65

  int dimtime = (75*dimming);    // For 60Hz =&gt;65    
  delayMicroseconds(dimtime);    // Off cycle
  digitalWrite(AC_LOAD, HIGH);   // triac firing
  delayMicroseconds(10);         // triac On propogation delay (for 60Hz use 8.33)
  digitalWrite(AC_LOAD, LOW);    // triac Off
}

void loop()  {
  for (int i=5; i &lt;= 120; i++){  // We use 120 as the lowest and 5 as the highest brightness setting. May be adjusted depending on the lamp used.
    dimming=i;
    delay(20);
   }
  for (int i=120; i &gt;= 5; i--){  // Same as above
    dimming=i;
    delay(20);
   }
}


They are using an interrupt as suggested by chrisl.
There is no flickering present with low dim values (using an incandescent bulb).
See this video for a similar board with the original version of the above sketch.

Note: If you want to integrate your own code into this example, it will likely not work as intended because the Arduino is busy with delayMicroseconds() most of the time



EDIT: The manufacturer of this module just released a library. Check it out on their github