It requires two Next Gen Stryd footpods paired together to offer left/right balance metrics. You don't need a membership to collect those metrics, and the two footpods pair with your watch as a single unit. The membership is required however if you want to take advantage of their Stryd Footpath software capabilities in PowerCenter.
Looks interesting, but I don't know how much of that data is actionable and if it will lead to performance improvements. But if you are a physical therapist or doctor, I'm guessing this data would be incredibly valuable.
Thoughts?