Programmers like the Bully Dog Triple Dog GT stay plugged into your trucks ODBII for "on the fly" programming. Downloaders connect thru ODBII temporarily just to "download" a program into the ECM/TCM. I have the Bully Dog Triple Dog GT, but I cannot tell you how well its tunes work because I only use it for gauges. Any program you download into your ECM/TCM will void your factory powertrain warranty and any programmer or downloader that make changes to your ECM/TCM will leave a footprint and will be found.