What is the meaning of "tanks al-salat" (reversing the prayer), and is it permissible to extend the "takbirat al-tashahhud" (proclamation of faith) between the two rak'ahs to inform a newcomer who missed the first rak'ah?

"Reversing the prayer" (tankis al-salat) means that the worshipper begins with the Taslim, then sitting and reciting the Tashahhud, then prostrating, then bowing, then standing, then saying the Takbir, and recites during the sitting posture and recites the Tashahhud while standing. This invalidates the prayer by consensus. However, it is authentically reported that there is a consensus allowing reversal in the case of someone who finds the Imam sitting or prostrating, so they begin with that, which is the end of the prayer. As for prolonging the Takbir, there is no objection to it.