Numbness in calves during seiza

When doing the calve exercises at the beginning of the series, I feel a lot of lactic acid building up in the calves. Generally not a problem, but because the next exercise is a seiza where we need to sit on the calves, they start to become feeling very numb - almost like there is no blood flow going on. It starts to become so painful that I can not finish the full 120s. Instead I need to sit up and shake the legs every now and then, disrupting the stretch.

What can I do to prevent this from happening?

Probably the soreness it is due to the hight intensity. You can consider to lie on the floor with legs in the air for 60'' and then begin the seiza.

I always have to pause the video and a do a quick 20second calf stretch after the 2nd and 3rd set of calf raises, otherwise I just cramp half way through each following Seiza. That seems to solve it for me. 

I have this happen too. I will stay standing and shake my feet out before getting into seiza. The feet up trick works good too, especially on the more intense days. Sadly, after six plus months, it still happens for me. I wish I could say it gets better. But that might be just me, as I have this problem when running too.