How does beforeMarshal event works?

Hello everyone,

I have two models Contents and Terms, where Contents HABTM Terms.
In my CreateContent form, I have built a structure as shown in the following
part of the request->data table

terms => [
    columns' => [
        'single_select' => [
            'id' => '3'
    'tags' => [
        'unlimited_select' => [
            'vocabulary_id' => '1',
            'names' => 'tag2,another,war,tag1'

That is because there is the possibility of picking a term from a set of existing ones (using <select> tag)
or typing terms (within an <input> tag) which creates new ones.

Since that format is not following the CakePHP conventions I want to re-structure the data.
I tried to use the Model.beforeMarshal event as indicated in the Modifying request data before building entities section on the Book but with no luck.

When I call the patchEntity($content, $this->request->getData(), ['associated' => ['Terms']]) method, the Terms are not patched at all. After digging and scratching a little bit the cakePHP code (classes Table & Marshaller I found out that the original data are passed to the corresponding methods marshal() and _marshalAssociation().

Does the beforeMarshal() modifies the actual data or not?
What am I missing in this context?

Thank you very much in advance