If you want to split maybe optional properties into different tables or collections. Every extension will be in a new table, where the unique identifier of each row is the main model instance id. For example:

var Person = db.define("person", {
    name : String
});
var PersonAddress = Person.extendsTo("address", {
    street : String,
    number : Number
});

This will create a table person with columns id and name. The extension will create a table person_address with columns person_id, street and number. The methods available in the Person model are similar to an hasOne association. In this example you would be able to call .getAddress(cb), .setAddress(Address, cb), ..

Note: you don't have to save the result from Person.extendsTo. It returns an extended model. You can use it to query directly this extended table (and even find the related model) but that's up to you. If you only want to access it using the original model you can just discard the return