What does a block tackle in soccer mean exactly? Below is good video demonstration of how to do a block tackle and not risk hurting your knee. In a block tackle you use the inside of your foot to win the soccer ball.
However, if you go in hard and your leg is out too wide then you can injury your knee - most likely the MCL (Medial Collateral Ligament). So it's important to have your body over the ball, keep your ankle locked, and not open up your leg too much. And your standing foot shoud be slightly bend and firm on the ground.
Take a look, women's national team player Christie Rampone demonstrates the correct technique.
And here's another video showing you the right away to do a block tackle in soccer. Don't forget to lean your shoulder into the tackle too.