After creating a basic twist, you'll notice that unless you hold the two sections of the balloon, it will tend to untwist itself, which is why you need to learn the lock twist.
The lock twist effectively holds the balloon's segments in place so you can continue to twist and form a sculpture. You'll also find that the lock twist forms the basis for such things as the head and ears of a dog sculpture, as well as the legs and body and more.
To start, apply two consecutive basic twists to create two balloon segments. You'll now have four balloon segments. To make this easier, try and create two small segments, as shown in the picture. When starting out, it's important that you try to get the two sections as close to each other in size as you can.
Notice in the picture how the hand has to hold all four balloon segments to keep the balloon from untwisting.