This is what was once added to C#: arr[^idx], when this ^idx is mapped to a special object, typically optimized then out. arr[^0] means the last element.
[^n] indexing is mapped to an 'Index' struct by Roslyn which can then be applied to any array or list-shaped type (it either has to expose Index-accepting indexer, or a plain integer-based indexer and Count or Length property. There really isn't much to optimize away besides the bounds check since there are no object allocations involved.
A similar approach also works for slicing the types with range operator e.g. span[start..end].
But I guess they wanted to iterate from the end back [-1] to the start [0], making it easy to implement a rotating buffer.