Below is a pic of some forms that were erected to contain and essentially "form" the new retaining wall and underpinning of the existing foundation wall. What you see straight on is the retaining wall which has forms on both sides held together by steel pins and what you see on the right that is supported by all of those 2x4's is the underpinning. The underpinning only has a form on one side and it is necessary to support it with 2x4's like such as the weight of the concrete can very well push the form away from the foundation and spill.
Now the pic above represents nearly 1/3 of the work that needs to be complete. When underpinning, it's important "NOT" to do everything at once otherwise you risk a total cave-in. So, we're are doing one corner with 6' of underpinning, we'll do the other corner with 6' of underpinning, then will mend the middle 6' between the two ends. The last bit of concrete work will involve pouring the floor.
Below represents the retaining wall that has forms on both sides. This leads perpendicularly to the existing foundation wall and underpinnings:
What you see below is the other end of the pic from above, essentially the T section where the retaining wall meets the existing foundation wall:
What you see below is where the new concrete replaces the area of the existing footing that had fallen free. This way the footing is repaird and the work is continuious through the room and provides the same structural integrity throughout:
What you see below represents the remaining dirt that needs to be removed and the footings that require underpinning (the other 2/3).