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"Had a bit of a brain wave. There I was, smashing some steel plates together, and I thought, 'yes, it's deadly!' But what's missing? What's missing? And I thought lots of sharp bits welded onto the flat bits."
It is then formally introduced by Wheatley, after he assumes power in the Enrichment Center, in a death trap against Chell and GLaDOS, after he decided that the common Panels were not as deadly and effective in killing her, despite that it could flip over Chell to make her fall to her death. After the duo escaped the first death trap, Wheatley repeatedly attempts to use the Crushers to kill her, but fails.
After regaining control over the Enrichment Center, GLaDOS then adapts the Crushers into the Mass and Velocity test course in the cooperative campaign, where ATLAS and P-body have to sacrifice one another in an attempt to disable the Crushers. It is then used for the last time in the Mobility Gels test course, located in the unnamed Co-op Test Shaft. As the title of the course suggests, the bots are to utilize the momentum of speed using the Propulsion Gel to send themselves across several Crushers.
Behind the scenes
Although it was not originally named, a moving ceiling with spikes was demonstrated in the earliest Portal trailer. This suggests that the concept has been thought over for its application in Portal 2.
Although Wheatley claims to invent them, two pairs of Crushers can be seen while Chell was falling down to the Incinerator Room. Also, in the non-canon informational video Aperture Investment Opportunity #1: "Panels", CEO Cave Johnson states that Aperture Science sells Crushers. It could be that Crushers had been made before, but were simply unused until Wheatley came into power; or that he essentially "reinvented the wheel", thinking he had made something new; or he simply lied about creating them to further boost his ego.