A normal shift is one that happens voluntarily, on purpose, and in safe environments. The more experience a Protector has shifting between forms, the faster it goes, and there are plenty of older Protectors who can shift in under a minute, although doing so can cause fatigue and joint pain.
Shock shifting is, as its name implies, a shift that happens under drastic conditions causing the Protector's body to go into shock. Since generally the canine form is hardier than their human form, this often grants them the ability to survive under conditions that might kill them otherwise. Examples include Protectors surviving avalanches, earthquakes, car accidents, and murder attempts due to shock shifting. There are circumstances where shock shifting works against them, however, because their canine instinct leads to panic, such as drowning or in a building on fire. Shock shifting is a major stress on the body, and can itself be dangerous in some situations. Usually a Protector needs 24 hours of rest and hydration to recover, but there are plenty of examples of Protectors shifting back into their human form within hours, and sometimes within minutes, of a shock shift.
For full explanation of the social/cultural aspects of bonding, ref. Bonding.