After this message:
Press ESC for .altboot
appears, the secondary boot loader reads in the root block of the partition.
This is the second block of a partition (or disk in the case of a floppy) and contains inode entries for /.boot and /.altboot, the two OS images that are available to load from the root directory.
The boot loader locates the second block, using the parameters for heads and sectors/track that were stored as part of the secondary boot loader when it was written to disk by the dinit utility.
The boot loader then loops for 2 seconds waiting for you to press a key.