The kettle stitch, which holds the tension along each signature and acts as the change-over for many bindings, is a persnickety little stitch until you get the hang of it. When learning the coptic or sewn-to-tape bindings a common problem is an incorrect kettle stitch that does not 'lock' the tension, or a kettle so successful it cinches the head and tail while the center of the spine bows outward. These diagrams illustrate how I execute the kettle stitch, along with a trick for regulating the sewing tension both along the spine and between the signatures.
On reaching the head or tail of a signature, I exit the last sewing station, make certain the thread is nicely taught, then clamp down with a forefinger on the fold about 1/2" inside the last sewing station. The thread (and tension) is held firmly by this finger, and the end of the signature pops up just a bit. That tiny bit of extra space will prevent the kettle from cinching too tightly. I hold this position until the kettle stitch is completed with my other hand, locking the thread in place.
To execute the kettle stitch while holding this posture may seem acrobatic at first, but it can be easily done using the technique recommended in Non-Adhesive Binding, Volume III by Keith Smith. Having made certain to place the sewing stations a distance from the head and tail that is shorter than the length of your needle, its point will emerge from between the signatures and can be pulled through the loop in one easy motion. To tighten the kettle stitch keeping that extra iota of space, pull the thread straight upwards. And finally, release the finger that has been clamping your signature down all this while.
Another helpful technique is the french link stitch, which regulates the tension across the spine (between the signatures) by linking each stitch spanning a tape to its neighbor. The french link stitch is an extra step, but the herringbone-like pattern it creates can be a lovely addition to an exposed spine.