The engine as you know has many intricate working parts, some work in tight tolerances, the pistons, cams bearings etc. The run in means that these components should not be overstressed for the first few hundred kms, perhaps for 1st sometimes till second service and oil changes. The whole idea is that the wear and tear damage in the initial stages should be minimized. Once everything "settles" after few hundred or thousand kms the engine could be pushed a bit harder. Initially best to not over-speed it and use proper gears always further reducing undue engine stress.