The AVR outputs are push-pull and the FET drivers are bidirectional. I.e. when a pin is OUTPUT HIGH it will let the current flow from an external source to Vcc when the source tries to apply more than Vcc to the pin. As long as the inductor current is low enough and the pins are kept OUTPUT no snubbing is needed: when both driving pins are driving to the same value they will short the inductor and let the current slowly decay dissipating the energy stored in the inductor in the resistance of the driver and the coil itself.
When the current through the inductor is too high (causes more than 0.5 V voltage drop in the pin driver) an external protection is needed. Diodes from schematic 2 add such protection when both pins are written LOW.
Both approaches cause the energy stored in the coil to dissipate as heat. When using a diode bridge as described in post #6 and configuring the driving pins as INPUT the stored energy will be returned into the power supply. Maybe this can extend the battery life a bit. But it may be also dangerous: primary lithium batteries may explode when recharged. I don't know if such small but repeated pulses may trigger the explosion.
