As the vacuum cleaner decreases the pressure inside the bag, the bag-person should feel a tightening or squeezing sensation on the body. Observers will notice that the bag conforms to the bag-person’s body. This happens because higher air pressure always wants to move toward regions of lower air pressure.
The squeezing sensation can be quite substantial. The squeeze is caused by the plastic’s reaction to the difference in the external air pressure outside the bag and the internal air pressure inside the bag.
When the bag-person first gets inside the bag, the external and internal air pressure are the same. At sea level, air pressure is usually 14.7 pounds per square inch (psi), 1 atmosphere (atm), or approximately 1 bar. You can see this in the diagram below (click to enlarge).
Depending on the vacuum cleaner, the pressure inside the bag can be reduced from 5–20%. When we tried this experiment at the Exploratorium, we averaged a little over 1 psi (about .1 atm or .1 bar) reduction in air pressure in the bag using a 12 amp vacuum—close to a 10% reduction in internal air pressure.
Once turned on, the higher external air pressure pushes the bag's outside surface towards the lower air pressure regions inside the bag. After a few seconds, the bag will come into contact with the bag-person’s body. However, there are also spaces where the collapsing bag doesn’t touch the person’s body, such as nooks and crannies where the legs are crossed, near the armpit, where the arm bends, or where the hand is holding the hose—even between the fingers. You can see this in the diagram below (click to enlarge).
Once the bag touches the bag-person’s body, the plastic wants to stays in place. External air pressure will force the plastic to stretch and wrap around various body parts where the bag is stuck. At the spots where it doesn’t touch the person’s body, the bag will try to force its way into the gaps. The stretching plastic causes the squeezing sensation around the bag-person’s body.