This bare-bones ticket rule will send an email to the Approver as soon as the Approver is set and the ticket is saved. 

Title:

Email Approver

Notes: Send an email to the Approver as soon as the Approver is set and the ticket is saved.
Frequency: on Ticket Save
Select Query:
select
  HD_TICKET.ID as TICKNUM,
 HD_TICKET.TITLE as TITLE,
 APPROVER.EMAIL as EMAILCOLUMN
from
 HD_TICKET join HD_TICKET_CHANGE C on HD_TICKET.ID = C.HD_TICKET_ID
   and C.ID = <CHANGE_ID>
 join USER APPROVER on HD_TICKET.APPROVER_ID = APPROVER.ID
where
 (C.DESCRIPTION like '%Changed ticket Approver from%')
or (C.DESCRIPTION like '%Ticket Created%'
and APPROVER_ID != 0)
 
 √  Send an email for each result row
Subject:

 Ticket [TICK:$ticknum] Requires Your Approval 

Email Column:

EMAILCOLUMN

Email Body:

Ticket: $ticknum - "$title" requires your approval.
Please open this ticket at:
http://kbox/adminui/ticket.php?ID=$ticknum