A balloon is inflated but not tied off. It is then pressed against an object and allowed to deflate. The rubber wraps around the object and makes it seem like it is now inside the balloon. The principle is related to Lubor Fiedler's Dental Dam Coin Penetration.
This idea was used with a deck of cards and a balloon by Ralph Cos, who performed it at the German national competition in 1987, see Joe Nex's report in Magie, Vol. 67 No. 7, Jul. 1987, p. 215. (The routine was inspired by Jay Sankey's “Airtight” from Richard Kaufman's Richard's Almanac, Vol. 2 No. 15, Nov. 1983, p. 137.) Cos published it with several handling ideas two years later in SimsalaThema, No. 40, May 1989, p. 20.
The principle was used in a Coin into Modeling Balloon called “Stretching the Boundaries” by Kenton Knepper on his videotape Klose-Up and Unpublished, 2000.
The application of the principle to Airtight made a comeback in the 2000s, starting with a handling by Román García that was performed by Luis Piedrahita in the Spanish magic television series Nada x aquí, Season 2 Episode 1, 30. Sep. 2006. The details were later published in García's book Nanomagias, 2010, p. 71. Further handlings were developed by Daniel Garcia and Dan White (“Pressure”, 2009) and Shawn Farquhar (“Inflation”, ca. 2011).