Check out this video to see if they are what you are trying to accomplish:

How to build cascading drop downs in the New Responsive Designer in Nintex Forms for Office365

​@bamaeric , Thank you, not exactly what I am looking for. I want to set the selected item of the dropdown in a rule. I will save the video for future reference though. 

Thanks.

Hi ​@jschmidt7,

In this example I have a choice single control configured as a dropdown and a yes/no control.
To set the dropdown value when the check box is ticked would look like this.
 

This was my mistake; it is actually a List Look up and not a single drop down. 

Hi ​@jschmidt7 

Currently, a rule cannot set the value of a list lookup control.