Como funcionam as co-rotinas / geradores de PHP

Https://gnugat.github.io/2014/04/30/php-generators-and-coroutines.html

function holy_grail_countdown($number)
{
    while (0 < $number) {
        $reset = (yield $number);
        $number = $reset ?: $number - 1;
    }
}

$c = holy_grail_countdown(5);
foreach ($c as $number) {
    echo "$number\n"; // outputs 5, then 2 and finally 1.
    if (5 === $number) {
        $c->send(3);
    }
}

Ele envia o valor 3 para expressões ((yield $number), mas parece que ele executa 2 iterações.

# 1st iteration
$reset = (yield $number); # reset=3
$number = $reset ?: $number - 1; = #number=3
# 2nd iteration
$reset = (yield $number); # reset=null
$number = $reset ?: $number - 1; = #number=3-1=>2