In Sanskrit Mudra means "seal". All mudras require that your hands be in particular positions (like as you say when you sit for meditation with your thumb and first finger together). It may not seem as if this would be an especially effective practice, but don't let the simplicity fool you

. Mudras have been used for thousands of years; there is little doubt the practice is effective.
The spiritual benefit from performing a mudra lies in the way the positions of the fingers put pressure on and activate your energy meridians. The way the fingers are placed, the amount of pressure applied, the length of time the position is held makes all the difference. Also, the point where pressure is applied must be precise, or the practice won't work. (It's a bit like a computer touch screen; if you don't touch the right spot, nothing (or the wrong thing) will happen.)
Mudras basically work the same way acupressure works along energy meridians. When you press upon certain points on the fingers and hands, you activate energy channels through which energy runs through your body. In meditation if you press your thumb and first finger together you form an energy loop, which allows the prana to circulate back into the body.
Hope this explanation helps.