The top diagram above shows the result of an over run brake telescopic section having had a sideways knock in a collision with a large vehicle when the trailer was stored.
The arm has been stretched and compressed on opposite sides and the elastic deformation is permanent. In the second diagram above the tow car is out of line with the trailer. If it is necessary to apply the tow car brakes in this situation the trailer will try to continue in the direction shown and in so doing exert a force on the telescopic arm which will tend to temporarily deform its shape very slightly ( as shown in the extreme case in the first diagram), with the result that the arm will not slide into the housing to apply the brakes. I have also given additional reasons (in other paragraphs) which also prevent the brakes being applied in this situation.
With electric pneumatic or hydraulic actuating mechanisms the brakes come on when the tow car driver depresses the brake pedal and so very much reduce the possability of the trailer jack knifing.
