As with anjali mudra, http://thenamastecounsel.com/anjali-mudra-prayersalutation/ reverse prayer stimulates the balance between the left and right hemispheres. Similarly, with all fingers touching another, a maximum number of nerve endings and acupressure (marma) points on the fingers and wrists are stimulated.
The stretch required for this hand mudra opens the heart and solar plexus (and sometimes throat) chakras. Reverse prayer, energetically, can boost the thyroid and provide emotional release making it a “feel good” pose, physically and mentally.
On a physical level, this mudra helps keep the spinal cord elongated, open the abdomen and chest to allow for enhanced breath work, and it stretches the shoulders and pectorals. It is practiced in the Ashtanga primary series in parsvottanasana (pyramid). It can also be added to tree balances, for a greater challenge. Those with less shoulder flexibility can opt for traditional prayer, or grab the forearms behind the back for a simpler frontal stretch.