"All in memory". I wonder how it got there without hda1 ?
The bootloader (example: grub) reads hda1 and copies bzImage and the initrd (tinycore.gz) to memory. At that point, the bootloader hands over control to the Linux kernel which uses the tinycore.gz initrd as the root filesystem.
It is just like booting up the tinycore ISO image from a CDROM drive.. The CD will not be mounted after the system is booted and after it is booted you are free to remove the CD from the drive.
In traditional versions of Linux (RHEL, Debian, etc.) things work differently. The boot loader stills loads the kernel and an initrd in to memory, but the main function of the initrd is to have the kernel modules necessary to mount the "real" root filesystem which is on a disk. Tiny core doesn't have a "real" root filesystem anywhere on disk to mount, its entire root filesystem is the initrd, and therefor is in RAM (initrd is short for initial RAM disk).