To underfill a verb is to provide it with fewer arguments than there are slots in its definition.
Using a transitive verb as though it were intransitive (Huogaı jí da.) or using an intransitive verb as though it were nullary (Loe ꝡo!) are both examples of underfilling.
The common consensus is that this is allowed, but it's not so clear what exactly it means, and how it comes to mean that.
In logic or mathematics, there's no sensible way to "underfill" a relation. "Twelve is divisible by three" makes sense, but "Twelve is divisible" does not. So when we turn "I hear it" into "I hear", what is really going on?
Verb families
One explanation is that, for example, huogaı is actually a "verb family" of three verbs in a trenchcoat:
huogaı2: ___ hears ___.
huogaı1: ___ hears.
huogaı0: Some hearing happens.
Depending on how many arguments we provide, huogaı selects a completely different lexical item.
This is tidy, but it doesn't gives us a predictable rule for what the intransitive version of a transitive verb means. In English, "I know" means "I know it" but "I eat" means "I eat something". Should this distinction be mindlessly carried over into Toaq if we define dua1 and chuq1 as above?
Implicit arguments
Another explanation is that when we underfill huogaı, the remaining slots are filled with some implicit argument. But which? All of sá raı, báq raı, ké raı, and a "vague definite reference" / pronoun seem to make sense in different situations.
Disallowing underfilling
An extreme idea is to ban sentences like Huogaı jí, forcing the speaker to say something explicit and specific like Huogaı jí sá or Huogaı jí hóq. This is semantically watertight but annoying.