Your 360 cannot be jtagged. The whole jtag mod is basically writing a replacement (modified) operating system to the 360 motherboard's onboard memory (nand). But to run this replacement operating system (freeboot) you need to start from a 7000 series or earlier dashboard, so very early NXE or any blades dashboard.
The details are a lot more complicated than that, and a jtag can do a LOT more than just play games directly off the hdd with no disk required. But the important thing is you would need a 360 with an old dashobard, and some soldering skills (or a qualified modder) to do the work required to "jtag" a console.
I hope that helps!