As per the API Documentation, it is a method that only updates the specified timestamps of the model with the current time. So in order to update the updated_at field:
product.touch
or to update the updated_at and designed_at fields:
product.touch(:designed_at)
Now, I have never used this method before, but I’d think it would be useful in a situation to “bump” a question (like on Stack Overflow) to the top of a search query without actually changing the contents.