Detailed explanation
Formal English uses inversion in several structures that do not begin with a negative adverbial. Place inversion moves an intransitive verb before a full noun subject after an initial place expression: At the centre of the courtyard stood an ancient tree. The structure is common in descriptive, narrative and ceremonial prose because it presents the setting first and introduces the new subject afterwards.
Place inversion normally occurs with verbs of position, movement or appearance such as stand, lie, sit, hang, come, emerge and appear. A full noun phrase usually follows the verb. Pronouns resist this inversion: Here comes the bus, but Here it comes, not *Here comes it*. The verb agrees with the postposed subject.
Conditional inversion omits if and places should, were or had before the subject. Should you require assistance means If you should require assistance and presents a relatively open or polite possibility. Were the evidence stronger means If the evidence were stronger and expresses a hypothetical condition. Had we known earlier means If we had known earlier and expresses an unreal past condition.
These forms are formal and concise, but they should not be mixed with if. Write Had we known, not *If had we known*. Negative forms need careful order: Had the team not intervened, the system would have failed. In a were-condition, were can introduce an adjective, noun phrase or infinitive: Were the proposal feasible; Were she the director; Were the system to fail.
Inversion affects information structure and register. Place inversion foregrounds a location or direction and often creates a visual sequence. Conditional inversion foregrounds the condition and can sound more diplomatic or authoritative than an ordinary if-clause. Use it selectively; excessive inversion can make prose artificial.
How to form it
Place inversion: initial place/direction phrase + intransitive verb + full noun subject. Conditional inversion: Should + subject + base verb; Were + subject + complement/to-infinitive; Had + subject + past participle. Do not retain if when using the inverted form.