Lagging refers to when the average water level of a lagoon rises, under the effect of the swell.
Causes and consequences of lagging
When this swell is strong, sea water penetrates the lagoon through the hoas (gap in the coral reef) faster than it can be removed through the pass (discontinuity of the reef).
The difference in water level then causes the lagoon level to rise and amplifies the exiting tide currents. This can also cancel out incoming currents from time to time.
Lagging can cause lagoon waters to flood an atoll. © EVS-Islands CC by 2.0