Serious question: where do downswings start?

[Karak567]

Honestly, is a downswing from your highest peak? Or is it from where you started your session?

For example, last night I ran up 2 buy-ins and then dropped 6 due to variance (it's heads up; super high variance is the norm). Is that a 6 buy-in downswing or just a 4 buy-in downswing?

I was down 4 from my starting point, but 6 from my peak.

I have always wondered this.