How in the world does the input predicate work

Trying to get the new input predicate working, running the latest snapshot
this is my predicate:
{
    "condition": "minecraft:entity_properties",
    "entity": "this",
    "predicate": {
      "type": "minecraft:player",
      "input": {
        "forward": true
      }
    }
  }
  

which is in fact detected by the game, and I'm running this in a repeating command block with:
execute if predicate huo.test:forwards run say hi

shouldn't this spam hi? whats going on