An Empty Rectangle is also known as a Hinge. In this example the hinge
is in box 2. It allows the blue cell in row 1 to bend around the
corner and 'see' the target cell.
Here's how it works:
The two blue cells have a strongly linked candidate
1. The target in this case is r9c4.
One of the blue cells must be a 1. Obviously if the bottom blue
cell is a 1, the slashed 1 can be eliminated.
The upper blue cell can indirectly 'see' the target cell (r9c4) via the
'hinge'. If the top blue cell is a 1, the other 1's in row one
would be eliminated, causing the 1 in r3c4 to be a 1 (it would be
a hidden single in box 2). That would eliminate the slashed
So either way, the slashed 1 can be removed.