فَيُصَلِّي بِهَذِهِ الطَّائِفَةِ رَكْعَةً وَسَجْدَتَيْنِ
Translation: The imam then prays one rakah, including its two prostrations, with this group.
This describes a two-rakah prayer, for example a shortened traveler’s prayer. The first group performs the first rakah with the imam.
Mentioning the two prostrations identifies precisely when the exchange of groups occurs: after the second prostration is complete.