Six Sigma methodology is often used in combination with Lean approach tools.
Lean Manufacturing or Lean production is Toyota’s Production System for achieving a progressive movement of material through the facilities without stocks, without losses, without any unnecessary resources, at minimal production cost. Lean Manufacturing is a strategy for maximizing value for customers and profit for the corporation by eliminating waste through every process step.

Lean Manufacturing defines seven types of waste:
• Overproduction (producing more than is needed or/and before it is needed),
• Defects (repair, rework, scrap),
• Unnecessary Inventory (excess inventory of raw materials, parts in process, finished goods, etc.),
• Inappropriate Processing (higher quality than necessary, extra operations, etc.),
• Unnecessary Delay (waiting for tools, parts, materials, operators, etc.),
• Unnecessary Transport (unnecessary transport of materials, parts, or finished goods between work stations or into/out of storage),
• Unnecessary Motion (any wasted walking or wasted motion to pick up parts or stack parts).
 
Synergy of Six Sigma and Lean Manufacturing allows improving both system's effectiveness and efficiency.

Why is complex use of Six Sigma and Lean Manufacturing so effective?

Six Sigma system is not aimed directly at increasing of process speed. Lean Manufacturing in itself does neither allows achieving a statistical controllability of the process, nor sets requirements to concept implementation form and infrastructure.
Six Sigma and Lean Manufacturing concept is synergy of Six Sigma tools oriented for process quality improvement and Lean Manufacturing tools served for process speed increase.