Booting without Bootmanager
Before being able to boot from another partition (another operating system), you have to select it in some way.
The are actually no standardized techniques for this. Without a bootmanager, you have to "activate" this partition in the partition table before booting another operating system.
Of course, this is quite awkward. That is the reason so many so-called "bootmanagers" are now available on the market. These programs allow users to select a partition and with that an operating system during booting.
The hard disk partitioner PartitionStar may be used to switch the active partition via command line options.