A pair of Shimano SPD cleats without clearance in black.
The difference between SH51 cleats and SH56 cleats is the number of directions in which you can release the cleat from the pedal.
SH51 cleats have a single direction of release; that’s to say you can only disengage from the pedal by pushing your heel out in a flat plane relative to the pedal, or by pushing it in.
The SH56 cleats, on the other hand, enable multi-directional release; as well as pushing your heel out or in, you can also roll your shoe on the pedal or pull up to release it.
The shape of the cleat is subtly different from the SH51 to enable this.