multi-select OR values to filter entries preface: i'm new to craft so may get terminology incorrect

basic idea: i would like to show entries of a particular channel which have multi-select field values which intersect with another set of values. e.g. given set ['a','c','e'] it would show entries with values such as ['a','b',c'] and ['e'] but not ['b','d'].

what i have done so far:

my basic ElementCriteriaModel for entries in the past is defined:

{% set results = craft.entries({
    section: ['mysection'],
    when: ' &lt;= ' ~ now,
    order: 'when desc',
}) %}

{% for entry in results %}
.... blah blah do stuff


and what i would like to do is add another criteria on there for the multi-select field. however, from what i am reading and have tried it doesn't seem like that kind of filtering is available. please stop now and prove me wrong if this is not the case!

i was able to filter matching a single option of the multi-select by using:

{% for entry in results if entry.myMultiSelect.contains('a') %}


but was unable to determine syntax to "OR" the results with another contains(). really not much documentation that i could find on that method.

alternately i possibly could use a search, but sounds like that relies on indexes that may get out of date which i would like to avoid. wouldn't be a huge deal, but looking for the "right" answer before i settle on brute forcing it by just looping through the multi-select values inside the loop to check for a match.
Not an expert myself, but you can try .contains('OR', 'a', 'b', 'c', 'e')

However, rather than first fetch all entries and then filter them, I would look into using relations when looking them up, so that the initial result only contains the entries you're after.

Have a look at https://craftcms.com/docs/relations to get started, and don't be afraid to ask for help if you can't quite figure them out (I found them a little tricky myself when first attempting to use them).