Bouncy seats are seats with suspension bases similar to those used in trucking. One sits on an adjustable (for weight) spring and the up and down motion is damped by a shock absorber. If you're car is bouncing along at 20 HZ you might be bouncing at 5 HZ - slower with a noticeable difference in onset. The seats can be locked so they won't bounce.

They were in some GenI cars, but not all - my '89 Raider did not have them, but I've replaced the original seat bases with suspension bases from the junkyard. I cannot rock the seats forward to get into the back seat - but I no longer have a back seat - replaced with more useful stuff.

I am told that there were some installed on GenII's. I have never seen one, but these may be ones:
http://cgi.ebay.com.au/ws/eBayISAPI...0828575&ssPageName=STRK:MEWAX:IT

On GenI cars, swapping is easy, all the bolt patterns match. I'm in the slow process of trying to install a GENI suspension base in my GENII Pajero. Much, much different - a huge PITA.