The meaning of Verbal

"From", when used with the verbs "form" or "derive", means the same as "come from"  or "related to".  In this sentence you need the preposition "from" to show the direction of the change, in this case "from" the verb to the new verbal form.

Verbal are non-finite verbs - they are independent of, and not limited by, the subject, as are finite verbs (is, are, am - these finite verbs have to change according to the subject/person/tense/number in the sentence).  

Verbals can take on the form of adjectives/adverbs/nouns, such as "Walking makes her happy." Here, walking is the verbal that takes on a noun form.  Verbals can be infinitive, gerunds, or present and past participles.

In your sentence, "formed" means it is created out of, or "from" - so, it's appropriate to say: Verbal : is the technical term for a word that has been formed from a verb, for example, a Gerund, Infinitive or Participle.