The 1980 Peter Lougheed version of "turn off the taps" was pretty much that. Alberta oil shipments to the rest of Canada were reduced during the legal battle with the federal government.
The 2018 Rachel Notley version of "turn off the taps" just involves switching the existing Trans Mountain line over to carry 100% bitumen. The net effect for Alberta would be more sales of bitumen in exchange for lower sales of crude oil and refined products... but they might be able to find buyers in Eastern Canada and the US for the crude oil and refined products not being sent to BC, because they're more marketable and in-demand. For BC, bitumen is of no use, because BC has no refineries able to process bitumen. All the bitumen that comes out of the pipeline would be shipped overseas.
The mechanics of the law involves requiring export permits for petroleum products, something Alberta already requires for LNG producers. They would simply issue permits for oil and refined products to be shipped south or east, while only issuing Trans Mountain Pipeline export permits to people exporting bitumen.
The law makes no reference to BC, and frames the purpose in terms of maximizing exports by making most efficient use of existing pipeline capacity.
-k