A soft catch simply slows down the absorption of energy in a falls.
When executed properly the climber will have a significantly slower stop then if the belayer just held the belay device locked off a...
I had thought about writing an article on belaying and soft catches, but I don't think I could do better than this.
http://homoclimbtastic.com/2012/05/20/better-belaying-the-soft-catch-aka-the-dynami...