In the room of jhal letters, there is substitution of jaś letters, when jhaś letters follow.,
A mute letter is changed to a sonant non-aspirate mute, when a sonant mute follows it.
This is the well-known rule of softening the hard letters. Thus लभ् + ता = लब्धा, so also लब्धुम्, लब्धव्यम्; दोग्धा, दोग्धुम्, दोग्धव्यम्; बोद्धा, बोद्धुम्, बोद्धव्यम् ॥
Why do we say झशि, 'when a jhas follows'? Observe दत्तः, दत्थः, दध्मः ॥,