Matched context for obstruct and prevent back

The page lists matched contexts for obstruct and prevent back.

Share - obstruct	Share - prevent back	Phrase
12.58%	22.16%	{obstruct or prevent back} flow [perform operation]
4.02%	16.84%	valve [does] {obstruct or prevent back}
0.90%	7.78%	{obstruct or prevent back} fuel [perform operation]
0.87%	4.86%	substantially [degree] {obstruct or prevent back}