They're kind of like car bumbers, the get destroyed, but expend energy in the process thus protecting the "occupants."
As for the gate flutter thing. I've never heard of this happening with people I know using screamers. That said, if it is a big concern, use wire gate biners, which have less gate mass and are less likely to move around with the stitching breaking.
Also, since the stiching breaks at much lower levels than the typical 6-8 Kn of an open biner, the main vibrations while the gate is potentially open are being absorbed by the stitching. What does the stitching break on a regular screamer - like 2 Kn?