panvn,
"Bridging" is just a general term for connecting an incoming call to an outgoing trunk. If you both have an OBi you can put each other in your "Circle of Trust".
If your uncle calls your OBi number, then he will get your auto attendant. He can then choose Option 2 and make an onward call using your OBi service providers.
If your uncle sets a speed dial in this format: pp(ob200123456*12345678912), where 200123456 is your OBi number, then he will automatically use your Primary Line (let's say GV) to send the call to number 12345678912. If he chooses speed dial slot 4, then he just has to dial 4# for the call to be placed. Such a call will be totally free for him.
It works both ways. For instance you can dial your uncle's OBi number and use the PSTN connected to his OBi110.
Much more can be said about how it all works. The basic principle is that the OBiTALK network joins all OBIs together for free. You can decide who you trust to have access to the service providers on your OBi.
Your decision now is what sort of Obis to buy. I suggest that your uncle will need an OBi110, so he and anyone he trusts has access to the local PSTN. This is not such an issue in the US, so long as GV stays free PSTN is not really required. You might want to consider an OBi100 or OBi202 depending on how many services etc you might need.
You may wish to read the first post here:
http://www.obitalk.com/forum/index.php?topic=3094.msg20656#msg20656