I was reading a driving book from the 20's and it had this manoeurvre in it.
You would do this manoeurvre when you need to turn the car round to go the opposite way.
When you have gone the wrong way, there is a blockage ahead or the Police close the road.
The left reverse has to be done slowly, if you do it fast you do not have time to see children behind you. The "slow walking pace" is an ideal speed.
You need to ask yourself SCLaP, is it Safe, Convenient, Legal & Possible.
Safe: are their children around?
Convenient: is the road very busy? Is it easier to drive round the block?
Legal: is it a One Way Street or a dual carriageway?
Possible: is the minor road wide enough?
The speed you need to do this is a 'slow walking pace'.
You may be asking, this is so we have time to process what we can see behind us, children are unpredictable and we need to see them, this is especially important when doing the bay park at a supermarket.
You are the obstruction and other road users have right of way.
There are two times that you have to stop, this is 1) just before you are about to turn left into the side road and the car is about to stick out more. 2) when you are at the most vulnerable position and blocking the road the most.
You also need to stop for any other vehicles as we are the obstruction.
This gives time and mental room for the driver of the other vehicle to decide to wait or go.
Drive back 3 or 4 car lengths trying to keep within 2 feet (60cm), 2/3 foot (20cm) is perfect from the kerb.
Drive on when ready.
The left reverse in the context of the driving test:
You will be assessed on three parts: 1. observation, 2. control and 3. accuracy.
They will not be impressed if you do this fast.
If you mount the kerb you will fail.
If you are heading towards the kerb and you think you are going to hit it, drive forward a bit and reverse again.