The electronic bobbin let-off unit keeps a constant tension from full to empty bobbin. Constant tension is essential to produce a tight leno selvedge regardless of the bobbin diameter. The bobbin let-off is mostly used for selvedge weaving, but it can also be used for additional fancy yarn threads with a different color or yarn count. Selvedges often differ from the body of the fabric in construction or weave, or both.

Therefore the selvedge threads are wound on separate flanged bobbins. Sometimes selvedges may also contain fancy effects or may have brand names or fabric descriptions woven into them. For all this a constant yarn tension is very important.