This solution works but is a bit complex. If you have a very long list of answer options in your first question (multi-select), then your choice filter syntax needs to be extremely long because you need to account for the possibility that a respondent will select a large number (or possibly all) of the answers in Q1. You end up needing something like:
selected-at(${Q1},0) OR selected-at(${Q1},1) OR .... selected-at(${Q1},N}
where N is the number of answers to the previous question - long and possibly error-prone, and a bit tedious to do if you have many such questions.
A quicker way to do it is to put the following in the choice_filter column for Q2:
selected(${Q1},name) = 'true'
Only answers checked in Q1 will appear as options in Q2.
Cheers,
Matt