Ushiro Goshi means back hip and is almost the opposite of Ogoshi. Here you slip behind your opponent and drop your hips below uke's while circling their waist with your arms. From there you drive up with your legs and thrusting forward with your hips, this will generate a large amount of lift and will cause uke to be easily lifted off the ground, you really need to get the thrust with the hips and bend yourself backwards a bit otherwise you're just going to pick uke up and put him back down on his feet. Once uke is airborne it's a pretty simple matter of just getting your body out of the way and guiding uke to the ground, as gently or forcefully as you desire.
As far as it being a counter throw, yes it is a good one. In fact:
Kodokan Judo by Jigoro Kano said:
Ushiro Goshi is used to counter a hip throw
That's the first sentence under this throw's description in the Jigoro Kano's book, Kodokan Judo. To me, a good counter throw is one where the tsukuri, or setup and distance is a position that uke will often enter while trying to setup a throw on you. Here, if someone tries to do an Ogoshi on you, they end up almost perfectly in position for an Ushiro Goshi, as long as you are quick enough to spot it. In fact, this is a pretty good counter for any type of front throw. The only problem is that if you have a good opponent, if they can't get the initial off-balancing on the setup for a throw, they won't follow through, they will switch to a different technique, which denies you the chance of countering the throw, and if they do manage to off-balance you, you are in a situation where countering becomes very hard.