Wherever possible, the IEG engine tries to delete data as soon it finds out that it is no longer relevant.
If an answer is explicitly modified by the user (through a regular question, a list-question or a set-attribute, but not through a custom function call), the engine detects if this answer is used in a condition expression, a list-question criteria or a loop criteria. If that is the case, the expression or criteria is re-evaluated and if it becomes false, the corresponding pages are removed and the associated data gets deleted without the need to go through the script to encounter the expressions or criterion.