The most universal characteristic of any defoamer is the fact that it is surface-active but highly insoluble in water. It has to be formulated so that it will be dispersed as tiny droplets, i.e. as an emulsion.
The surface-active nature of defoamer causes it to spread very rapidly onto any air-water interface that it encounters. This is especially the case if that interface is already covered by the types of surface-active materials that tend to stabilise foams.
Defoamer contains particles that pierce the surfaces of foam bubbles, causing them to collapse when it spreads at the interface.