What is the technique called when a straight part is taken at the scalp and a small section of hair is positioned with foil?

The technique referred to in the question is called slicing. Slicing involves taking a straight part at the scalp and selecting a thin section of hair, which is then placed into a foil. This method allows for precise application of color to specific sections of hair while maintaining a clean and defined look. It is particularly useful for achieving solid hair color or highlights because it results in less blending than other techniques, making the styling outcomes clear and distinct.

This practice stands out when compared to other techniques such as weaving, which involves interlacing strands of hair to create highlights or lowlights, and balayage, a freehand painting technique that adds a more natural, sun-kissed appearance. Shading typically refers to a technique used to add depth through color variance but is not associated directly with the use of foils or the defined method described in the question. Slicing is thus the most accurate technique for the scenario presented.