Wet ground and loose gravel can cause the dog to loose footing and slip.  Going up incorrectly, a loaded cart can drag a dog down a hill.  Going down incorrectly can have a cart run up on the dog and cause the brakes to fail resulting in serious injury.
	A hill that is too steep can put undue pressure on a dog in either direction.
	Asking a dog to hold a loaded cart on a hill in either direction pointed in the incorrect direction can cause undue stress on the dog either pulling or pushing a dog down a hill.
	I’ve been on hills so steep that even empty carts were rolling down a hill into a pond.  Unfortunately, the organizer of the event insisted that we position the carts parallel to the slope instead of perpendicular to the slope.

To protect against these problems, hills should always be tacked.  This concept is not new nor is used only in dog drafting.  The concept of tacking, sometimes called switchbacks, is used in sailing, windsurfing, biking, skating, and driving.  In sailing and windsurfing, the operator sails or surfs at an angle towards the wind, not directly towards it.  In biking, skating, or driving, it is easiest to go up a hill at 45 degree angles instead of dead up the hill.  That method reduces the steepness of the hill and makes it easier to go up or town.  If you want to test this theory, just grab a pair of skates and try to go straight up or down a steep hill.  Make sure you wear a helmet and protective gear first.  You’ll quickly learn that you can bleed off speed by tacking down the hill or ease the steepness by tacking up the hill.  In driving, cars with short clearances that need to make it up or down a steep incline should approach at an angle either way.  Here’s some photos to illustrate the concept.
 
If you’re familiar with Lombard Street, here’s a photo of cars tacking down this 27% grade hill.  Read more details here. 

Tacking a hill (up or down) is pretty self-explanatory and straight forward.  You traverse a hill by going up diagonally at a comfortable angle.  You will be traveling more distance to go up the hill, and it will be at a less steep slope.  More importantly, it will be safer!  I also go up curbs and small hills with my own dogs at angles.  When leaving your harnessed and hitched dog or cart (without the dog) on a hill, make sure that they are positioned perpendicular to the hill.  You don’t want the cart or dog to be overcome by the weight of the cart when on a steep hill to cause a runaway. 

Here’s a photos of some cars parked perpendicular to the slope (correct).  If you don’t do this in San Francisco on some extremely steep hills, you’ll have some runaway cars.