You go active duty and work for a couple years in your rating and then switch to the reserves.
It's good to do if you're certain that you want to be in the reserves.
I wouldn't do it if you were thinking about "trying out" active duty to see if you like it and then switch to the reserves if you don't. Out of the many FTS people I've met that have tried to stay on active duty because they loved it were denied. They had no choice but to switch to the reserves and they were sad that they couldn't further their active duty time.
It's good to do if you're certain that you want to be in the reserves.
I wouldn't do it if you were thinking about "trying out" active duty to see if you like it and then switch to the reserves if you don't. Out of the many FTS people I've met that have tried to stay on active duty because they loved it were denied. They had no choice but to switch to the reserves and they were sad that they couldn't further their active duty time.