What is V2? It simply means that a verb must follow the first part of a sentence such as a noun phrase or other constituent. In other words, the verb is in second position in the sentence.
For example, the English sentence "Sometimes he comes late" does not have the same word order in other Germanic languages such as German, Dutch, Swedish, Danish and Norwegian. In Swedish the sentence is "Ibland kommer han sent". The word-by word translation is "Sometimes comes he late". The verb is in second position. The English sentence "At three o'clock I was here" does not have the same word order in German. In German the same sentence is "Um drei Uhr war ich hier". The word-by-word translation is "At three o'clock was I here". The sentence "Tomorrow we travel to London" is "I morgen rejser vi til London" in Danish. The word-by-word translation is "Tomorrow travel we to London". We see that V2, a verb in second position, is regular in all Germanic languages other than English.
However, English does have some sentences which follow the V2 pattern. They occur with the adverbs "rarely", "hardly", "seldom" and "never". The regular word order is one which starts with the subject but to add special emphasis, it is possible to put the adverb in subject position.
The sentence "She is never late" has a regular word order. However, it is possible to move "never" to the front and say "Never is she late". Other similar examples are "Rarely are they late", "Hardly did she eat" , "Seldom do they visit" and "Never have I seen them". Notice that with the examples "hardly" and "seldom", it is necessary to insert a do-verb in second position. The reason is that with these adverbs, it is more common to use a main verb rather than simply an auxiliary when they are moved to subject position. With main verbs, inversion is not possible, thus the auxiliary "do" must be inserted.
Why does English have V2 only in a few cases? I suspect that English used to have regular V2 as is the case in the other Germanic languages. It may have lost regular V2 due to the influence of French. In French, having the verb in second position in the sentence is not necessary. This can be seen in the sentence "Demain je vais a la piscine" which means "Tomorrow I'm going to the swimming pool". As in English, French places the subject "I" after the adverb "tomorrow".
The sentences with adverbs such as "never" and "rarely" which place a verb in second position when they begin a sentence must be from an early period in the development of the English language. In this earlier period of the English language, V2 must have been regular. Thus, we can say that the sentences with these adverbs reflect an earlier stage of the English language in which putting the verb in second position in the sentence was required as in other Germanic languages.