Assertion is a way of asserting that something will happen, is sure, is affirmative, is positive, assures, etc. Like negation, it's commonly used in sign language.
The characteristics of assertion are all or some of the following: head nod or nodding repeatedly; firm expression (brows burrowed, eyes squinted); and, tight or closed lips.
This sign WORRY is a regular verb without assertion nor negation.
Now, this verb "worry" has an assertion. It can be used in such context as "I'm definitely worried" or "She's surely worried!"
Another example with using assertion is this HAVE as in "do have", "definitely exists", "indeed there!".
This sign "HAVE-to" or "MUST" comes with assertion. "You have to" (as if with no choice).
Sometimes, assertion is used throughout a sentence or a predicate (verb phrase).
Notice that those animated images above are GIF-animation. That was from the 2000s long before the emergence of video on the Internet. What an artifact! Now one more example with the recent video (below.
ASL (glossed as): "IX-she/he HAPPY [assertion]". English translation: She/he is happy.
Related posts: Negation