Promote: Workflow for approval

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We recently noticed that managers are able to promote their employees. We would like to include workflow that if a manager does submit a promotion that our HR group is able to be notified and must approve/deny before it moves on. Looking at workflow I can see there is workflow for promotions in the Management Employment workflow as well as Promote workflow. Management Employment is set to bypass and Promote is not.

What is the difference between Management Employment and Promote workflows?
What does "set to bypass" mean in a workflow?
Has anyone added a workflow to include an HR approval before it is moved on as information to others, and if so, do you have examples?