X=[1,2,3;4,5,6]
X(2:-1:1, 3:-1:1)
2:-1:1 is the list of integers [2 1]
3:-1:1 is the list of integers [3 2 1]
So that is a call to index
X([2 1], [3 2 1])
When you index with vectors or arrays, MATLAB produces an output for each combination of entries. So it is going to produce an output for X(2, [3 2 1]) and an output for X(1, [3 2 1]) . In turn, X(2, [3 2 1]) would produce an output for X(2, 3), X(2,2), X(2,1)
So the output would be
[X(2,3), X(2, 2), X(2,1)
X(1,3), X(1, 2), X(1,1)]
