Joining the Salaah while the Imaam is coming up from ruku

If you went into ruku while the Imaam was coming up from ruku, then it will be seen that if the point you reached while going down was such that had you spread your hands towards your knees, it would have touched the knees and similarly the Imaam while coming up from ruku, if he reached the point that had he spread out his hands it would have touched his knees, then in this case you had got the ruku with the Imaam and you got the rakaat. Accordingly your salaah was valid and you will only have to make up two rakaats after the Imaam makes salaam. However, in the case where the Imaam while coming up from ruku reached higher than the point we had explained and then you began going down into ruku (i.e. the Imaam reached the point that had he spread out his hands, it would have not reached his knees), then you did not get the ruku with the Imaam and you did not get the rakaat. In this case, after the Imaam makes salaam, you will have to perform three rakaats. If you are doubting whether you got the Imaam in ruku, then you should look at the dominant portion of your mind and if you are unsure, then regard it as though you did not get the Imaam in ruku.

And Allah Ta'ala (الله تعالى) knows best.