When the material enters a larger cavity with a high flow rate jetting might occur.
While the melt doesn’t hit a wall, a snake-like cord is build until a wall is reached.
The cooling down of the cord surface prevents a good merging between the cord and the new entering melt.
This leads to a reduction of the stiffness and a visible mark.
Slow injection or a bulging wall directly behind the gate will prevent the jetting effect.