Frequency of once a week only- why?

How does performing the exercises twice a week or more impede your progress?

Good question - It does seem hard to believe that once a week is optimum

The stretch series are really intense and may need recovery on their own, especially in the beginning.

Take super-unstudly me for example - I'll typically be quite sore from stretching, especially after FSs and MSs. Because of that (and because it makes scheduling and thus consistently following the course easier for me), I even do FSs and MSs once every two weeks, alternating between them weekly and I still progress - perhaps even better than when I was trying to do each weekly.

I got very sore the first time I went through the Stretch courses (especially FSs, holy calves!), but at this point seldom feel much of anything regarding soreness.

However, I think the thinking is that we are ideally already stressing the structures via our HS and Foundation work strength and mobility multiple times per week, so adding more stretching volume (which is stressing the connective tissues more than the muscles themselves) would possibly result in improper recovery and increased risk of injury.

That's my theory anyhow. I could be way off.

Once a week works for long-term, sustainable progress. I would imagine diminishing returns on doing them more times per week, but can't imagine any real harm unless you are super gung ho; of course to do them more often you would not be able to stretch as hard as if you had a week to recover, so you might do it twice at 75%of normal, which in practical terms is going to be a lot less than 3/4 as productive as a single harder session; personally I'd rather invest the time for a single session and keep progressing week to week.

I look at these as recovery and re-alignment, and the course elements as 'real' mobility work, but this is probably just because my passive flexibility is much much better than my active so i don't find these stressful

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Thank you so much guys for the replies, makes sense

When you combine all of the program you won't have much time to add more in. Foundation, handstand, stretch, tumbling and limbering exercises will keep you busy enough.