The name comes from the piking movement used by the shrimp to swim. The move is a defensive move that can also be used to set up offensive movements.

To shrimp, you must be on your side, keeping your elbows tight to your body. Dig into the mat with both feet and extend you legs to move your hips along the mat until they reach a plane in alignment with your head.
Ideally only your shoulder and feet should be in contact with the mat.

It is also a good idea to practice with only a single foot providing the drive. This mimics the situation in which your opponent may be restraining one leg.
Get used to all types of this movement: double leg shrimp, bottom single leg shrimp, and top single leg shrimp.

Backward shrimping

Shrimping Reverse Bottom Leg

Sideways shrimping

The art of shrimping by by Keith Owen ( The proper way to shrimp)

Reverse Shrimping