OnParticleTrigger runs every frame?

I'm having some difficulty understanding Particle System triggers. I want to have the "Inside" callback only execute when my Collider is inside the particle system.

My initial idea was just to throw a print statement into the OnParticleTrigger() function, but this seems to run the print statement every frame:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 [ExecuteInEditMode]
 public class ParticleTriggers : MonoBehaviour
 {
     ParticleSystem ps;
 
     void OnEnable()
     {
         ps = GetComponent<ParticleSystem>();
     }
 
     void OnParticleTrigger()
     {
         print ("inside"); // why does this run every frame?
     }
 }

I then thought maybe there's a problem with my collider and particle system and they're always colliding, but this isn't the case. If I change the color of the affected particles (see this page) then the particles only change colors once I'm inside the particle system.

So am I misunderstanding OnParticleTrigger? Does it run every frame? This seems to defeat the purpose of calling this feature a "callback".