The fly half deliberately runs at the gap between the flanker and the fly half. As one or both step to make the tackle, the fly half passes into the space.
Either the blindside winger (11 in the picture) or the inside centre (12) can run into the space.
This is particularly good against a "drift defence" where the opposition 7 targets your 10 and their 10 your 12 and so on.
If there is a shortish pass from the scrum half, this is not going to be that effective, mainly due to the small gaps between flanker and fly half.
This article is from Match Day Tactics, which will help you to banish some of the worries of match day through modern methods and techniques.
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