Tim Watts wrote:

Load up 24xNumLEDs bytes into RAM and let it rip. Do
whatever at the end to cause the registers to be latched to the LED
drive outputs.

They latch one-by-one not all at once, e.g starting from an idle state

the first LED (well all of them) is listening for the combined
clock/data to start,

it receives the first 24 pulses without echoing them,
and latches those as its colour,

after that it receives and echoes every pulse it hears to the next LED
until it sees the pulses have stopped,

when it goes back to waiting for a new colour for itself.

so an unbroken sequence of pulses will "fill up" consecutive LEDs with
colours, but a break in pulses starts back again with the first LED.