There are various forms of the sign of the cross. Western Christians are accustomed to using the open hand with all fingers stretched symbolizing the five wounds of Christ.
Eastern Christians by bringing the thumb, index and middle fingers together call to mind the Trinity, the three persons in unity. The other two fingers bent signify the human and divine natures in Christ. Therefore, faith in the two natures in Christ and the Trinity are expressed, the latter doubly since the words also express this faith.
Another form which is used in blessing by bishops of the Eastern Churches is depicted in a sixth century mosaic, Christ Panocrator. Here Jesus is seen blessing with the index and middle fingers together and raised. The raised thumb is apart from the two fingers. The little and ring finger are folded into the palm.
Triple repetition of the sign of the cross represents the Trinity. It was a common practice to repeat prayers, especially three times. We repeat the Kyrie three times at Mass. Bishops bless with a triple form of the cross, one at the Father, one at the Son and one at the Holy Spirit.
Bowing is a common gesture for prayer and a sign of respect. It was commonly used when the name of Jesus was heard. A more profound bow can substitute for a genuflection. Both bowing the head and genuflecting are meant to convey to us our littleness before the greatness of God.
My Ukrainian Catholic friends tell me that they have never noticed others kiss the joined thumb and fingers, suggesting some cultural groups may do so. However, I could see it as a normal practice as a sign of love and respect for the Trinity, which this configuration represents.
It seems that, until the Middle Ages, all Christians touched the right shoulder imitating the blessing motions of the priest who faced them. I haven't found any indication of when or why Western Christians changed.
However, an early text in Old English by the Bridgettine nuns (founded about 1346) reveal some reasons for the Latin practice. The hand goes first to the head for Christ came from the Father who is the head.
The left shoulder indicates Christ's coming down to earth to suffer and die. Lastly, the right shoulder represents Christ rising and ascending to the Father's right hand.
Whether they use one form or another, all Christians express faith in the Trinity by their words and in the Redemption by their actions. This gesture is also an act of consecration and a prayer of petition.
We touch our forehead, heart and shoulders, giving God our mind, heart and actions. We ask God to be in our mind and heart, helping us to perform right actions and forgiving us for our failures.
The small sign of the cross has very early origins in Christianity and it was repeatedly recommended and constantly made so that by 200 AD, it could be said: "We Christians wear out our foreheads with the sign of the cross."
Christians are marked with a cross at Baptism and Confirmation as a sign that we belong to Christ. The cross is a constant throughout our daily lives as we bear it with, for and in Christ.
We are repeatedly anointed with it in the Sacrament of the Sick, especially on our deathbed. For Christians, the cross is a glorious sign leading us to resurrection with Christ and everlasting life with Trinity.