To master kettlebell swing technique, thread a towel through the handle and grip it close to the bell. If you attempt to lift the weight with your arms instead of powering the movement through hip extension, the towel will go slack and the kettlebell will deviate, providing you with immediate feedback.

noli's read

Kettlebell swing technique is scientifically based on explosive hip extension (the hip hinge), not an active pull with the arms, as confirmed by biomechanical analyses (notably the modeling observation study by McGill et al., 2012). Using a towel is a classic motor learning trick, initially popularized by kettlebell expert Pavel Tsatsouline. Physically, the towel acts as a flexible link: any attempt to use arm strength breaks the alignment and causes the kettlebell to dip or oscillate uncomfortably. While there are no randomized controlled trials (RCTs) specifically measuring the effectiveness of the 'towel swing,' the use of external physical constraints to guide a movement's trajectory is validated by a strong consensus among strength and conditioning experts. This advice is therefore well-founded, as it exploits simple physical laws to force the body to naturally utilize the posterior chain.

the sources

No named source held this one up — that absence is part of the verdict.