harness lines...
Harness lines are your power transfer, so their position and length will affect rig performance.

Finding the right position for the harness lines on the boom is a question of balance.

Lines too far forward will limit speed, keeping the sailor too far forward on the board and the back arm becomes fatigued.

If the lines are too far back, the forward arm becomes fatigued.

Try sailing with no hands!. If the lines are in the correct position, you will be able to sail for some moments with no hands!.

If you reach for the boom first with your back hand, the lines should be moved back.

If you reach for the boom first with the front hand, the lines should be moved forward.

Lines which are placed further apart will give the windsurfer a feeling of greater stability.

Wide lines (30cm+) comfortably harness the sail's power and are generally better suited to beginning windsurfers.

Harness lines placed close together on the boom will make the power transfer very direct,  demanding greater attention be paid to to the rig´s relation to the wind.   Speed-oriented windsurfers usually opt for lines set close together.

Harness line length also influences performance. 28 to 30 cm from the harness lines center to the boom is a good average harness line length.

Short lines limit speed potential. Bodyweight is the power generator. Bodyweight too close to the rig, and maximum power is unavailable.

Overly short lines (less than 25cm max from boom) limits the reaction time as the windsurfer encounters changing wind and sea conditions.

Overly long lines (more than 32 cm) may cause arm fatigue and rather wet windsurfing as the sailor makes frequent contact with the water.

Outhaul tension will affect harness line position.   Soft (powerful) set will prefer the lines farther back.   Hard outhaul set (flat for strong winds) will bring the correct harness line positions forward. 

A loose fitting harness affects negatively harness function and windsurfing performance. Keep your harness tight!.