Answer:
It is important when doing exercises such as deadlift or squat to always keep what is called a "neutral" back. This is when you arch your back. If you do this, you do not have to worry so much about bending your knees. Yes, the "experts" tell us to keep the torso straight up and bend the knees when lifting but the true secret is to keep a neutral back to avoid back injuries--it doesn't matter if you stay straight up or not as long as your back is arched back.