SteezOnSkis12Can't this ruin the shape of the ski?
Yeah it would ruin the skis performance and shape if he did it a reasonable amount.
Like Poikenz said bouncing on your skis between two blocks will break and crack the fiberglass and epoxy as well as putting an unnatural amount of strain/flex on the core of the ski which would probably deform the profile of the ski.
OP if you don't give a rats ass about the performance and durability of your skis and just want a set of noodles go ahead and bounce on them a few times. If you do care about how long the skis will last then the only way to make them softer without causing any significant damage, is to spend a lot of time doing butters and flexing them while you're actually skiing.
In what way are they to stiff?
If they feel to stiff when you're buttering it's probably more to do with your technique and how well your boots fit. I haven't had issues buttering stiff full camber skis before, it's just the way you commit to it.
I like to ski with loose underpants or boxers on so my balls can swish around and chill.