OK.

Speeds are added together, or not, depending on what you are measuring the speed against.

Imagine yourself on a train. The train is heading forwards at 50mph. You walk through 1 of the carriages at 5mph in the same direction the train is travelling. Relative to the motion of the train, you are travelling at 5mph. But relative to the ground outside the train you are moving at 50+5=55mph.

Then imagine you roll a ball forwards as you are walking. You roll it at, say, 10mph. The ball's speed relative to the ground outside the train would be 50+5+10=65mph. Relative to the train it would be 5+10=15mph, and relative to you it would be 10mph.

The same principle applies to those mad lumberjacks who do log rolling. As the log rolls, they try to move in the opposite direction at the same speed. You therefore get x+(-x)=0mph, so the lumberjack should remain stationary.