Or rather, we should be talking about the signed volume of paralleloppipets in the sense described in the determinant video, using the right hand rule.
Wrapping your mind around this concept is going to help consolidate ideas from linear algebra, like the determinant and linear systems, by seeing how they relate to each other.
So that means we can solve for why by taking the area of this new parallelogram in the output space divided by the determinant of the full transformation.
The goal here is to find a value of Lambda that will make this determinant zero, meaning the tweaked transformation squishes space into a lower dimension.