Present Participal Phrases are whole phrases that work like an adjective, that is, they add some extra information about a noun, for example in I saw the children playing soccer as soon as I came home. The extract playing soccer is an example of Present Participal Phrase because it is describing the children in the sentence.
Therefore, in Jamming too much trash into a garbage bag, John saved several trips to the curb but had to carry much heavier bags, the extract Jamming too much trash into a garbage bag... is the present participal phrase descrbing John.