Loose Knees = Loose Jerks
Stand up and hang out for a moment. Now consider the muscles around your knees - are they tight and fighting to keep your knee from suddenly bending under the weight of your body? Probably not. We stand on a passive knee lock that doesn't require much muscular work. This is handy, as it would get pretty exhausting to be that active at all times just to stand still.
The problem occurs when we try to initiate a jerk from this position. If you begin your jerk dip from this passive knee lock, there will be a moment of slack in which you're essentially falling before your quads in particular catch up and begin controlling your downward movement. That abrupt drop will usually result in the bar losing contact with your shoulders, your weight shifting forward, and the rhythm of the dip and drive to be disrupted.
Before you begin your jerk, unlock your knees and create tension in your quads. This doesn't mean bend your knees significantly - in fact, it's ideally not really even a visible movement to observers. It just means you've shifted from a passive knee lock with loose quads to a straight knee with tension on the quads and your balance settled in perfectly on your heels while maintaining full foot contact with the floor.
From this position and state, you can initiate a smooth dip in which you're able to maintain your balance and position, and stay completely connected to the barbell. Make this one of your mental checks before you begin the jerk, and you'll see and increase in control and accuracy immediately.