I have some doubts about testing the effect that a decrease in value of x (independent variable) has on y (the dependent variable)

I have two observation point for variable x, so I can calculate the slope or simply the difference in value of x across the two observation points and I regress this difference on y.

I would like to have more information, however, about such a decrease. For instance, if starting with an high value at observation time 1 and decreasing down to a certain level has a stronger effect than the simple decrease?

Any idea on how to operationalize this?

Thank you very much in advance

All the Best