The s of stambh is changed into to ṣa after the upasarga (Preposition) ava in the sense of 'support' and 'contiguity'.,
The word आलम्बनं means \support, refuge, that upon which any thing depends or leans\. आविदूर्य means \the state of not being विदूर or far off, i.e. to be contiguous\.
Thus अवष्टभ्यास्ते \He remains leaning upon a staff\ &c, अवष्टभ्य तिष्ठति ॥ So also in the sense of to be near, as अवष्टब्धा सेना 'the army near at hand'. अवष्टब्धा शरत् ॥ See 5.2.13.
Why do we say \when having the sense of support or contiguity\? Thus अवस्तब्धो वृषलः शीतेन 'the Sudra is afflicted with cold'.
The present sutra is commenced in order to make the ष change even while the preceding letter is not इ or उ ॥,