I apologize for not showing you the actual interface, and just grabbing a random figure out of the paper. but hopefully both will help with the context.

for this tool, it actually is the lengths of the steps on the line that the kids have to draw themselves (at first), and once they've internalized that a basic feature of the model is that the steps are equivalent, the tool allows you to create fixed meter steps when presented with the second algebra problem (which is what is meant by reverse scaffolding). level 3's "stretchy" refers to proportionality, I think.