Waggle Momentum Needs To Start The Approach
Every jumper needs to go through a funny waggle in order to get going into the approach. This happens after the jumper visualizes the jump and before the jumper begins the straightaway part of the “J”.
The purpose is to allow the high jumper to establish consistency. The jumpers also reduce tension.
The movement is similar to the movement that happens before a golf swing. That means a golfer addresses the golf ball, takes a few wiggles with his or her wrists, then starts the swing.
The waggle can have many different forms and funny movements but its purpose is to establish a rhythm and ease tension before the jumper starts the straightaway.
In general, the purpose of the waggle is to prevent tension from building into the athlete’s body prior to starting. Tension is a huge killer of the start and movement prevents tension. Most instructors, including myself, would advise that small movements are typically better than large movements, and it is important to do the same thing every time before starting the approach. 
Speaking about this, leaning back heavily is the most preferred technique used by jumpers.
Olympic Champion Derek Drouin walks calmly into his mark to start the straightaway.
World Champion Marya Kuchina leans backward heavily and starts her straightaway.
I’ve also seen other jumpers pretend to catch bouncing balls before their start.Embed from Getty Images
The Legend Speaks
Tom Watson, world-famous golfing legend says “having a waggle before you start is important to avoid tension and establish good rhythm” (https://www.golfdigest.com/story/tom-watson-waggle).Embed from Getty Images
Every jumper should do the funny waggle in order to get his or her rhythm correct and to eliminate tension. The waggle is fun to watch but ultimately critical to the jumpers consistent approach starts.