There isn't a trigger specifically for a checkbox field changing. There's a general trigger for profile updates that you can use however and then condition for your checkbox field. See the below on how to do this.

Global
Triggers: onAfterUserUpdate, onAfterUpdateUser
User: Automatic
Access: Everybody
Conditions (custom value)
1: [var1_FIELD_NAME] Not Equal To [var3_FIELD_NAME]

This will allow to both frontend and backend. See the below for explanation of trigger locations.

Backend = onAfterUpdateUser
Frontend = onAfterUserUpdate

Be sure to replace FIELD_NAME with the actual name of your field. This will cause your auto action to fire when your checkbox field changes value. So either from unchecked to checked or checked to unchecked. If you want it to be more specific like only fire when the fields value changes to checked you'd add the following condition in addition to the above.

Field: FIELD_NAME
Operator: Equal To
Value: 1

You're not supposed to put that exact string into Custom Value. It's describing the following condition setup.

Field: Custom > Value
Custom Value: [var1_FIELD_NAME]
Operator: Not Equal To
Value: [var3_FIELD_NAME]