Ibn Majah

Benefits and Means:

➊
Standing up straight after bowing is called "qawmah."
Some of the supplications recited at this position have already been mentioned in Chapter 18.
Sitting between the two prostrations is called "jalsah."
Its remembrances will be mentioned in Chapter 23.

➋
Qawmah and jalsah are just as essential parts of the prayer as bowing (ruku) and prostration (sujud) are necessary components of the prayer.
The Messenger of Allah (sallallahu alayhi wa sallam), while alerting the Companion who made mistakes in his prayer, said:
"...
...
Then bow until you bow with tranquility, then raise your head until you stand up straight, then prostrate until you prostrate with tranquility.
Then raise your head until you sit with tranquility.
Then prostrate until you prostrate with tranquility.
..." (Sahih al-Bukhari, al-Adhan, Chapter: The command of the Prophet (sallallahu alayhi wa sallam) to the one who does not complete his bowing to repeat (the prayer), Hadith: 793)

➌
The method of sitting between the two prostrations is to spread the left foot and sit on it,
and to keep the right foot upright.
The method of sitting in the final tashahhud is that the left foot is taken out from under the right foot
and one sits on the ground.
See:
(Sahih al-Bukhari, al-Adhan, Chapter: The Sunnah of sitting in the tashahhud, Hadith: 828)