Many Zerg players will expand very early and get two bases up and running even before they have a spawning pool.
This can be a very bad thing to be up against as a Terran player since it means they will have a better economy than you almost right from the start.
But since you play Terran there's a way you can delay his early expand for a very long time with this tactic and it won't cost you any minerals.
Start your base like normal with a supply depot at 10/11 and a Barracks at 12.
After the SCV is done building the depot, rally him to your opponent's base to check out what's he's doing.
It's important that you scout early so you can find him (if it's more than a 1v1 map) and get in position.
Once you're SCV gets into his base, check it out and make sure he's got a spawning pool being built.
If it's done before your Barracks is complete, you may be up against a player that is about to rush you, so get a wall up fast to prevent from losing quick.
After you check out the Zerg base, go out to his natural expansion and build a bunker in the way of his Hatchery as soon as you can.
When it's done you should hop back up into his base to see what he's up to.
This will force the Zerg to make zerglings before getting his expansion down, which can throw him off if he doesn't have a back up plan ready.
As soon as your SCV sees zerglings pop out at his base, be ready to salvage your bunker (but don't do it yet!).
If he only makes 4 zerglings it will take over 15 seconds for him to kill your bunker, so when it's at around half life start salvaging (hotkey: S) This takes good timing so don't get discouraged if you lose the bunker.
After some practice you'll be able to salvage the bunker, returning your 100 minerals, and will have delayed the Zerg expansion by a long time, forcing your opponent to get behind or change strategies in the early game which can allow you to pull ahead.