Very often the Scrum under Waterfall exist in organisations. It might not exist formally, but it can be required by contract or by law. The Scrum can be part of the Agile experiment.
The Scrum under Waterfall be because it’s practically for the organisation.

What will be the same/different using Scrum under Waterfall?

But be aware, during Waterfall delivery: 

Waterfall has poor communication / misunderstandings
Lack of transparency
Detachment from reality
Slow feedback / No feedback
Nothing is Done

So you need to do following things:

1: Work in Sprints

The Waterfall can be scheduled for example 12 months. If one Sprint last 30 days, you will have 12 Sprints.

2: Avoid ‘Earned Value’

The task can’t be done for 85.6 or 98%. The task is done or not done!

3: Focus on Done

Create a Definition of Done (DoD), work with DoD

4: Testing, testing, testing

Test throughout Sprints, start to design your tests at the beginning of project. Test until the end

5: Get Frequent Feedback

Gather feedback early. Show the increment before it’s actually due

What is “Waterfall”?

Waterfall is a plan-driven project management in software development. So normally you work with Gantt charts & Microsoft Project, you have Start & end dates and Project phases. In Waterfall, development phases have force defined order: first you write Requirements, than you Design and Develop your application and at the end coming Testing and Deployment phase.

Waterfall vs. Scrum