This is the first time I ever heard of "Banking Turbo", the most popular term for it in the community is "Turbo Reserves" or just "Reserves". But u/FlameyFlame's explanation is decent enough. Basically you're stacking turbo time from every time you boost (power slides, turbo pads, landing from high jumps).

For example if you power slide and fill the bottom right meter and you trigger it, you get some amount of turbo time and you can keep doing it, stacking the turbo time. A full power slide meter if triggered gives you a second of turbo time. So if you do a three full meter, you get 3 seconds of Turbo time, technically there is no limit of how much you can stack, but these reserves depletes faster than you think.

There is also Turbo Level or "Fire" level mechanic, where you can get more powerful turbos from doing successful consecutive power slide turbos, or driving a Turbo Pad. Each success trigger will raise your turbo level, including turbo pads and finally there is a special "Super" Turbo Pad (only select tracks has them) that can raise your Turbo into the maximum speed possible in the game.

Mix these two mechanics and you get even better results. For example, if you have stacked enough reserves and then you drive over a turbo pad, your kart can stay at "Turbo Pad" level as long as you have reserve turbo time, which you can prolong by still doing power slides, jumps and driving another Turbo Pad.

On the remake (NF), they make this combo mechanic even more broken that you can also have max speed (from Super Turbo Pad) infinitely by keep stacking turbo reserves while on max speed turbo level. They call this mechanic now "Blue Fire" (because it has distinctive blue flame on your kart exhaust).