What is Expanding Square search as per IAMSAR ?

EXPANDING SQUARE SEARCH

• Most effective when the location of the search object is known within relatively close limits.
• The commence search point is always the datum position.
• Often appropriate for vessels or small boats to use when searching for persons in the water or other search objects with little or no leeway.
• Due to the small area involved, this procedure must not be used simultaneously by multiple aircraft at similar altitudes or by multiple vessels.
• Accurate navigation is required; the first leg is usually oriented directly into the wind to minimize navigational errors.
• It is difficult for fixed-wing aircraft to fly legs close to datum if 5 is less than 2 NM.

In other words we can explain this as :

This search pattern is most effective when the location of the search object is known within close limits. Here the search point commences at the datum position. This pattern is often used by small boats when searching for persons in the water or other search objects with little or no leeway. This procedure must not be used by multiple aircraft at similar altitudes due to the small area involved or by multiple vessels. It is difficult for winged crafts to fly legs close to datum if ‘S’ is less than 2m, and accurate navigation is required; the 1^st leg usually being oriented directly into the wind to minimise navigational errors.