If you run BeOS as the only operating system on your machine, you’ll notice that the Bootman menu imposes an unnecessary 5-second minimum delay as it waits for you to make a choice from the boot menu. While it would be nice to have a 0-second option, there’s a good workaround.
Go into DriveSetup and change the active partition to the BeOS partition you wish to boot from. Then, from Terminal, type:

writembr [devicename]

For example:

writembr /dev/disk/ide/ata/0/master/0/raw

Note that you only need to specify the devicename under R4. Under R4.5+, the correct device is probed for automatically.