Confinement is the physics phenomenon that quarks cannot be isolated. The color-charged quarks are confined with other quarks by the strong interaction to form pairs or triplets so that the net color is neutral. The force between quarks increases as the distance between them increases, so no quarks can be found individually.
... one end of the axis to push against the upper rim of the groove, while the other end of the axis pushes against the lower rim of the groove. While the axis is slipping ...