This manoeuvre should be carried out during routine driving, on a general main road which would normally carry light to medium traffic flow, therefore it is not necessary to deviate from the test route, into a side road or housing estate. It is not always necessary for the candidate to encounter traffic whilst completing this manoeuvre however carrying this out on busy roads or busy times of day, should be avoided due to the excessive length of time it would take to complete the exercise. The chosen road should allow the candidate plenty of opportunity to pull up on the right on a straight section of road with clear visibility well ahead and behind.
Whilst driving on a suitable road, and at a safe location, the candidate should be asked to pull up on the right when it is safe to do so. Once stationary, the candidate should then be asked to reverse for two car lengths before being asked to drive on again. If the candidates begins to select a location which is not suitable, then the exercise should be aborted before moving across to the right and attempted later in the test.
The candidate should show proper care for the safety of other road users while pulling across to the right, reversing and moving off.
Should the candidate ask the examiner to assist when reversing or moving off to re-join their side of the road, they should be informed that the purpose of the test is to prove their competence to drive it without the help of a supervisor, and that they should carry out the exercise as if they were unaccompanied.
The examiner must not get out of the vehicle to direct or assist the candidate.
If a vehicle pulls in front of the candidate prior to the completion of the exercise, the examiner should take control of the situation and advise the candidate to reverse back further to allow a clear view ahead. If a vehicle blocks the candidate from reversing, the manoeuvre should be aborted and a manoeuvre attempted later in the test. If this situation causes the candidates view to be severely restricted then the examiner should offer some assistance.
The manoeuvre ends once the candidate has stopped reversing and selected neutral.
The above is taken from The Driving Examiners Manual. https://www.gov.uk/guidance/guidance-for-driving-examiners-carrying-out-driving-tests-dt1
Where appropriate it has been reformatted and edited so it only refers to the driving of ordinary cars (class b).