From http://www.flickr.com/photos/53951927@N06/sets/72157627862581330/with/6231291712 and http://www.ebay.com/itm/TOUR-FRANCE-Time-Trial-Road-Bike-ORBEA-1989-Campagnolo-Delta-GHIBLI-Dura-Ace-/290620246466 as of 2012/08. Photos copyright by Ronald Hagenauer. Text on the latter page says the bicycle was ridden in the 1989 Tour de France and the crank was reportedly made at about the same time by a German maker. A label on the crank shows "Made in Germany by DKT" but has no date.
Presumably the inventor's idea is you store energy during the earlier part of the downstroke, then get it back again in the later part.
There are at least two big problems with that idea:
First, muscles use energy (and get tired) just resisting a force. A mechanical device does no "work" holding a load, but muscles take energy. Try to lift and hold a heavy weight, you will get tired — from the holding, not from the lifting. Similarly, for energy stored in the springs, you wind up doing the work twice: once to compress the spring, then again later while you fight against the spring as it expands.
Second, you have to work against the spring when the leverage is against you. Suppose there is a constant force trying to turn the pedals back. The closer the pedal gets to the bottom of the stroke, the worse leverage you have, so the harder your leg has to work to resist the force. It is unclear just how bad the effect is on the Interdrive crank, but it may take "doing the work twice" and turn it in to "doing the work three times"!
In some ways, this crank simulates the bad effects of pedaling-induced suspension "bob", but without the benefits of suspension.
Note the crank has three springs, one with an adjuster nut. That one appears to preload the other two springs, so under light pedaling there is no spring compression. That should mean the crank wastes energy only under vigorous pedaling.