on_dig only activates the specified event for blocks/tags not listed in the destroy_speeds section.
"minecraft:digger": {
"use_efficiency": true,
"destroy_speeds": [
{
"block":

{ "tags": "q.any_tag('stone', 'metal', 'wood_pick_diggable')" }

,
"speed": 2
},

{ "block": "minecraft:ice", "speed": 2 }

,

{ "block": "minecraft:sandstone", "speed": 2 }

],
"on_dig":

{ "event": "ihg:break" }

The event "ihg:break" will not activate from the tags stone, metal, wood_pick_diggable and the blocks minecraft:ice and minecraft:sandstone, only blocks not specified like dirt will activate ihg:break.