He is asking what is meant by being at ease in prayer; does it mean being at ease in one’s heart, and is the prayer invalidated if that is not attained, because it is an essential part of the prayer?
Question

What is the meaning of tuma'neenah in ruku? How can I determine that I attained tuma'neenah, what is the feeling? I see a person attains rest (tuma'neenah) normally in the last point of releasing his air from mouth when the joints of the body & mind both feel rest. Am I correct? What should be done if one misses rukn like tumaneenah, straightening backbone in ruku, sujud due to following Imam as following Imam is also obligatory. If in every rakah he misses rukn due to this reason what should he do? The missing can be for not getting enough time what he personally needs due to slowness of his brain to attain tuma'neenah or compact ruku, sujud of Imam. It needs to be mentioned that He tries his best to complete the rukn on time but can't complete.Will he pray in congregation as he misses some rukn regularly for following imam? What he will do in Jumuah prayer ? If you say that you need to fulfill that rakat in which you missed some rukn after imam's salam then I will say in 4 rakah salah I miss rukn in each rakah (that rukn is tuma'neenah). In this case will I repeat 4 rakah? Or I am excused for not fulfilling rukn due to my illness or trying my best to fulfill the rukn but can't or imam's compact timing of ruku?
Praise be to Allah.Firstly:
What is meant by the being at ease that is an essential part of the prayer, without which the prayer is not valid, is stillness of the limbs and pausing, even for a moment. According to some scholars, that should last for as long as it takes to recite the obligatory dhikr.
So when the worshipper leans forward to bow, and pauses for a brief moment, and his limbs become still, then he has attained being at ease.
If he remains still for a while, until he says “Subhaan Rabbiy al-‘Azeem (Glory be to my Lord the Almighty)” once, then he has attained being at ease according to the second scholarly view, which says that it should last for as long as it takes to recite the obligatory dhikr.
But if his bowing is like a pendulum, meaning that he bows and then rises without even pausing for a moment, then he has not attained being at ease, and his prayer is not valid.
Secondly:
Being at ease is not an action of the heart, so it is not the same thing as khushoo‘ (proper focus and humility in prayer), or feeling of calm with or without releasing the breath (breathing out); rather it is what has been explained above, namely pausing with stillness of the limbs.
If one who is praying behind an imam moves so slowly that he lags behind his imam, his action is not acceptable, even if he does that in order to focus his mind and attain comfort in his heart.
If the imam is clearly moving so fast that he is not fulfilling the condition of being at ease in his prayer, and he does not pause so that his limbs become still in each essential part of the prayer, then the prayer offered behind him is not valid, because he is not fulfilling one of the essential conditions of the prayer (namely being at ease in the prayer).
But if his speed is relative, in the sense that he is being at ease for the length of time that is required, then it is permissible to pray behind him, and the one who is praying behind him must follow him in his movements without lagging behind.
If you lag behind him only a little, in order to attain being at ease as is required, then there is no blame on you for that.
Moreover, you have to strive to attain both things: following the imam, and being at ease in your movements.
Please see also the answer to question no. 219808 – in particular, the third issue discussed there.
And Allah knows best.