How to get the handles position or influence

Sounds like you might be trying to do something clever with acceleration?  It might be easier (no pun intended) to use the velocity function.

transform.position.velocity

This gives you the velocity (change in position over time) of a layer., so you could use it to trigger something based on the layer's speed.  

Apologies if this isn't what you're after.  These websites are excellent resources for documenting expressions:

• https://ae-expressions.docsforadobe.dev/objects/property/
• https://www.plainlyvideos.com/after-effects-expressions-library/expose-layer-velocity