I have noticed that their pole is too short and they stand back too far nullifying most of the actual downward pressure irrespective of the apparent hard work they are applying. Also note that boy has a hole in the knee of his jeans. Probably the result of his praying too much and his dad spent all their money on curling and can't afford new ones. Proper brushing can be as important as skip rocks. Brushing cleans the rock's travel path.
Some brushes are adept at picking up lint, brush hairs, and water impurities.
The round relief hole has rounded edges and does not alter my longitudinal sliding forces since with my slide I follow the rock with a straight foot and my knee slider is of the conventional flat style. Rounded outer edges of any slider will minimize pebble shear. Hard brushes can shear uneven pebble tops which is not good except in the first two ends. This elusion is noticed after an ice-skate makes a trail in the ice. The rock also rides in the valleys between pebble.