A feature of lower security padlocks - this is where the locking bar only engages on one side of the shackle, leaving the padlock more vulnerable to hammer, pulling and twisting attack.
A single locking padlock is easily identified as it will have only one notch cut into the open end of the shackle.
Higher security padlocks use a locking bar that engages with both sides of the shackle, this gives a much better protection from hammer, pulling and twisting attack.
A double locking padlock has notches on both sides of the shackle.
The highest security padlocks (like the Kasp 180 and 190 series) use this locking mechanism. Instead of bars holding the shackle closed, hardened steel ball bearings are pushed outwards to lock into round indents on the shackle. This type of locking mechanism is extremely resistant to hammer or pull attack.