In order to avoid being blocked, defensive lineman must be very aggressive with their hands. If they can keep the offensive lineman from getting their hands on them, then they can utilize their various pass rush technique to get pressure on the quarterback. If they allow the Offensive lineman to get their hands, then the the O-lineman may be able to steer them where they want them to go.
In the clip below two drills are detailed that will help defensive lineman to keep the offensive lineman from getting their hands on them. The clip below is from Football Tutorials. For more great videos and playbooks click the link to visit their website or go to their YouTube Channel.
The video has sound, so please make sure that you have your sound on and that you have access to the site (Some schools block access to YouTube)