This one will require you to select the Bendy Mutation and stretch back-and-forth in quick succession.
To select the mutation, stand in front of the Kinect and wait for the Bendy symbol to appear on the computer screen.
If you are playing with two people, either person can select this mutation but only the primary player (the one signed and recognized as Player 1) will earn the achievement. To select the symbol as Player 1 stand to the left until the sign is completely filled green. If you are Player 2 stand to the right until the sign is completely filled green.
Allow a few seconds for Mutation Station to create the mutation and you will start to wobble from side to side slightly. There are two ways this can be done.
First: The key here is to start stretching your arms from side to side and tilt your upper body from left to right. Feel free to add a twist to get an extra effect and the achievement should unlock.
Second: Facing sideways from the Kinect start moving in a bowing position to an upward position until your projection is flailing from side to side.