In the war against oppression on the Harijan, a theme was that of the 'root and branch removal of untouchability.' Symbolic or rather the entering wedge in this respect was to be the throwing open of all temples to Harijan.
To cut up a large thick log of wood, the first step is to make a cut, put a wedge-shaped block in it, and bang on it. This often opens up a split. "Entering wedge" is a metaphor for making a huge job easier by using such a wedging approach.