The Preset Amount slider is a new beta feature in Camera Raw 14.0 releasing at MAX 2021.
To try the effect, use the Preset panel to apply a preset to a photo, then adjust the slider to reduce or amplify the effect of that preset.
The 'beta' label means that the feature is still under development. Please use this forum thread to provide feedback about this feature.
Thank you!
Rikk Flohr - Customer Advocacy: Adobe Photography Products