Fall on (one's) knees verb phrase
To go down on one or both of one's knees in order to express respect, awe, penitence, subjection or reverence
The monks fall on their knees and pray in the early morning.
People are falling on their knees and fold their hands to pray.
She fell on her knees and cried quietly.
He fell down on his knees in awe.
The verb "fall" should be conjugated according to its tense.
The origin of this expression is not clear.