In the presence of the Lord Jesus, we touch our right knee to the floor as a sign of respect.
When we greet a bishop, we may take his hand, kiss the ring which is on his right hand and touch our left knee to the floor, also as a sign of respect. When he passes us in procession, we may genuflect in the same way.
The Lord does not expect us to cripple ourselves, therefore a profound bow can be substituted for a genuflection.
We genuflect upon entering and leaving St Mary's: because the Blessed Sacrament is reserved in the tabernacle. When we receive Communion and are stepping out into the aisle, we may genuflect.