Bradshaw Leigh breaks down why the LA-3A has no attack, release, or ratio controls, by explaining exactly what lives inside the T4B optical attenuator. The T4B contains two key components: an electroluminescent panel and a pair of light-dependent resistors (photo cells) that together form a voltage divider, the same basic circuit as a fader on a console.
The peak reduction knob is simply a gain control for an amplifier that drives the brightness of the electroluminescent panel. As the panel gets brighter, the photo cell's resistance changes and gain is reduced. Because photo cells were designed for light meters and street lamps, not audio, their response characteristics are inherently unpredictable in this context.
That unpredictability is where the character comes from. Attack time is 1.5 ms or less, but release can range from 60 ms to 5 seconds depending on how hard the cell was hit, similar to how your eyes take longer to adjust after coming in from bright sunlight. The ratio also shifts with program material, starting as a soft-knee compressor in the first 10 dB of gain reduction before transitioning into limiting behaviour.
The limit/compress switch is less a literal ratio change and more a shift in the operating range of the attenuator. The result is a compressor that responds differently depending on the source, making it worth experimenting with rather than dialing in by the numbers.