In the Asgard mode (or base game), you will notice that the wild symbol appears on reels 2 and 4. When a stacked wild symbol appears on reel 2, you will activate the thunderstorm relocation feature. The reel is frozen (covered in the field) and re-spins once.
The stacked wildfire on the 4th reel triggers the Lightning Free Spin function. The Thor hammer rewards you with 2 to 5 wilds on a rotating reel and re-spins it once. In my opinion, this is more profitable than the thunderstorm remodeling feature, because 4 or 5 wild can bring you a huge victory. The two features described above look very honest and don't have that much reward. However, the real big win will come from the Rainbow Bridge function.