In [this comment](https://woodworking.stackexchange.com/questions/2484/methods-of-jointing-without-a-jointer#comment4924_2484) on this question on [jointing without a jointer](https://woodworking.stackexchange.com/questions/2484/methods-of-jointing-without-a-jointer) it was stated that:

> 
> there's a trick where, if you plane two edges at once, they'll fit together even if they aren't perfectly square to the face
> 
> 
> 

How does one go about doing that?