A prepositional phrase acting as an adverb modifying the nonfinite verb 'using' in that nonfinite clause...which I presume is attached by a comma to a main clause which you have not supplied.
Using the same sentence, but in the present tense, makes it clear that the sentence is not well written.
Conjuncctions join two words, two phrases, two clauses or two sentences. Wouldn't this mean that 'but' needs to join the prepositional phrase 'in the present tense' to another prepositional phrase?