Turning borders are recognized by their unique design where the border fabric seems to curve or turn around the saree's edges.
Have you seen a loom? The weft of the fabric is strung to the loom and the warp is woven across by taking the bullet back and forth. Most borders have a straight line, which mean that the warp bullet has to be moved back uniformly. Can you imagine the complexity of weaving a border which is curved?
Each point on the border is calibrated and the overall effect requires a high amount of precision as to when the warp is turned back, so as to create this unique effect. Hence these sarees, especially handloom ones are super intricate and difficult to weave.