This is from one of the new gas assesment questions,you have to log on and work out gas pressure drop through pipework
Personaly think they are barking mad,probably thought up by some trunkated splinter group
A old plumber I know had on of these
The idea is that every month that every job goes with no problems and all customers give you no grief you add a straight coupling other wise you add bent fitting depending on how bad its been
But then again thats why we loooove plumbing,never a dull day,well last Thursday was quite dull and last Monday was a bit but apart from Thursday and Monday always a challenge