“The tav is made up of three lines [which]… represent Torah study (thought), prayer (speech), and the performance of good deeds (action).
Because it is the last letter and thus the culmination of the alef-beis, the tav represents someone who is perfect on all these levels. However, the condition of being ‘perfect’ can also result in arrogance.
Therefore, the yud in the lower left-hand corner completes the tav to lend it humility. When a person knows he is perfect and that he has ended his service by fulfilling his level or destiny, he must arrive at the yud of humility. We see this in the design of the tav.
When Abraham was ninety-nine years old and essentially perfect, G‑d told him to circumcise himself. G‑d said, If you will circumcise yourself (also symbolic of the removal of the ego), then you will be ‘perfect.’
Why? Because if one is perfect in his own eyes without the aspect of humility, there is no true perfection. One can only be perfect when one circumcises his ego.”