Blending ṣād toward zāy
شمإشمام الصاد زاياً
ishmām al-ṣād zāyan
Distinct from the ishmām of pausing: here the ṣād itself is given a voiced quality, midway between ṣ and z. Ḥamzah's two transmitters read ٱلصِّرَٰطَ this way. Qunbul and Ruways go further and substitute a plain sīn.
Example
ٱلصِّرَٰطَ
al-Fātiḥah 1:6
Khalaf and Khallād blend toward zāy; Qunbul and Ruways read ٱلسِّرَٰطَ with sīn; the rest keep a clean ṣād.