The subtract node moves the range of the mask. The multiply controls the contrast.Multiplying higher than 1 sharpens overall. The add then offsets the range once more to bring it back to the 0-1 range, and the clamp ensures it is set to the 0-1 space. This is an optimization technique that gives us more control over the mask than a power node.
This is a more expensive version (100 instructions) of the built in node (98 instructions).
This is slightly cheaper then the built in node at 97 instructions.
By combining the data structures, we can reduce operations by remove 1 Multiply and 1 Add operation. This creates a cheaper calculation that does the exact same thing.
Texture Sampling (reading a texture) is the most expensive instruction that can be done.