It is technically a blend of 2D particle system with soft-body dynamics. A force can be applied between every two particles for maximum rigidity. To increase softness, threshold can be lowered, revoking forces between distant particles to simulate deformation. A Microsoft Kinect is used in Legion to capture audience body gestures once every second. Every body blob is essentially a group of interconnected particles with invisible constraints, which are gradually revoked along with the body blob rising and reaching the top. The blob will be removed once its softness maxes out.
The installation is programmed using Processing.