An audio CD is mostly arranged like a single continuous recording. Tracks are added on top of this via the Q subcode channel that gives information about the current location and the ToC stored in the lead-in area (also using the Q subcode channel). In the ToC, each track will have one more indexes that points at a specific location on the disc by minute, second and "frame" (represents 1/75 of a second, basically a sector).
If a CD is properly following the Red Book standard, index 0 will point to a 2 second pre-gap of silentce and index 1 will point to the actual start of audio of the track (additional indices are allowed, but not common). The purpose of the pregap is to make life easier for less sophisticated players that aren't able to seek to a precise frame on the disc. They just have to be able to hit a 150 frame region. However, just because the standard says the pregap is supposed to be 2 seconds and silent doesn't mean it actually has to be. Players generally don't care and by the time the format was popular, even inexpensive players could seek precisely. This allows you to stick audio data before a track that will be skipped by the player when it's trying to seek to that track. If you stick it before track 01, it will be skipped even when just playing the disc through unless you rewind.
The key for a hidden track at the beginning is that players usually start playing track 1 from index mark 1 (1.1) rather than index 0 as with continuous play through all subsequent tracks. The lead-in area for 1.0 is a holdover from grooved phonorecordings never meant to be played. It's a way for the primitive hardware of early CD players to acquire the start of the data stream in a safe area that doesn't have to be faithfully reproduced.
Some players permitted you to skip back from 1.1 to 1.0 to hear the lead-in as a hidden pseudo-track. Typically this was only possible with hardware index nav. buttons rather than the track nav. buttons, further obfuscating the presence of the hidden track.
The other means of "hiding" tracks is to have a bunch of short silent tracks until you get to track 99 (inconvenient to reach on a player without numeric track entry) or to have a long section of silence starting on the last track from index 1.
>Typically this was only possible with hardware index nav
Holding the previous track button would "rewind" playback and get you into the pregap on all the CD players I remember using, but these would have been late 80s models onwards.
If a CD is properly following the Red Book standard, index 0 will point to a 2 second pre-gap of silentce and index 1 will point to the actual start of audio of the track (additional indices are allowed, but not common). The purpose of the pregap is to make life easier for less sophisticated players that aren't able to seek to a precise frame on the disc. They just have to be able to hit a 150 frame region. However, just because the standard says the pregap is supposed to be 2 seconds and silent doesn't mean it actually has to be. Players generally don't care and by the time the format was popular, even inexpensive players could seek precisely. This allows you to stick audio data before a track that will be skipped by the player when it's trying to seek to that track. If you stick it before track 01, it will be skipped even when just playing the disc through unless you rewind.