the gritty details go in the model. even a simple contact form has lots of gritty specific details: names of fields, specific validation rules, etc.
the controller is the boss - validate this form! it does not say how to validate it, and it doesn't need the details of each field in the form. if the form validates - the controller calls the next view. if it does not validate - then the controller shows the form again and maybe passes specific error messages.
the validation details for the contact form - put that in a model.
the emailing of the form: the content of the email, taking the form values and putting it into the email, who the email goes to, the details of actually sending the email - that is all work for a model.
what does this gain us? when your client calls in a year and says - change the text in the email - you won't be hunting through a bunch of controller code. you will look at your model names and know exactly where to go. separation of concerns is not just for building - its for the inevitable changes that happen over time.
EDIT
yes the business logic should be in the model. so like for a contact form the business rules might be - we ask for name and address - and we require the email address and phone number. the validation rules then fulfill the business rules.
then six months later your client calls and says - well people don't like us requiring the phone number - just make the email required. you have put those validation rules in your model - therefore it is only that specific model which needs to change. the controller does not change at all! and since you separated the form validation rules from the sending of the email into separate models - if you make a mistake when changing the validation rule - its much less code to look through and debug.
the other way to look at is - how specific and 'granular' is the method? the more specific it is, the further from the controller it should be. the controller methods should rarely change because of changes to business rules.
so when i said the controller orders "validate this form" - i just meant that the controller is calling the validation method that is in the model, and then its getting back true or false, no matter what changes happen to the form over time.
