Crew Effects can be triggered on Crew Effect Spaces, which are locations marked by circles on Improvements that are installed Below Deck on a player's ship. Not all Improvements have Crew Effect Spaces, and those that have can work in different ways as described on the Improvement card.
When a Crew member is moved to a Crew Effect Space that does something that isn't an enabler or a bonus for something else, the effect happens immediately and the Crew member who moved there becomes Fatigued. However, if the Crew member already was Fatigued, the effect isn't triggered. Fatigue wears off at the start of the Player Phase, so this effectively limits each Crew member to triggering one Crew Effect per turn.
Improvement
Improvements are a type of Upgrade cards that can be installed in available slots in the Below Deck area of a player's Ship Board.
Improvements have different functions, and may have spaces marked on them; there are three types of such space: crew effect spaces (also called "circular" effect spaces), square effect spaces and rectangular effect spaces. Crew effect spaces require a Crew member mini placed there to trigger an effect. Square effect spaces contain some ship bonus value, for Improvements with such effects. Rectangular effect spaces can have other things than Crew members, such as enemy minis, placed in them.