The passuk says – ki shem Hashem ekra havu godel le’lokeinu – when I call in the name of Hashem, render greatness to our G-d
Alternatively, this passuk can be transalated – give a thumb to our G-d, but then the kashe is – why would you give a thumb to G-d?
The way the decision was made which cohen would get an avodah in the beis hamikdash was that the cohanim would stand in a circle and each raise a finger. The cohen in charge would stand in the centre and count round in a circle looking at the fingers to know which cohen he was up to. The cohen that he finished on got that avodah.
The mishna says in Yoma – ein motziin egodel bamikdash – The cohanim may not present a thumb to be counted becuase then they could also hold up their small finger and be counted twice and therefore have double the chance of getting the avodah.
This applies to a human in charge of allocating the avodah – however Hashem does not make mistakes therefore havu godel lelokeinu – you may present your thumb to Hashem when coming to daven shemone esrei which reminds us of the avodah.