A B rush needs to be well timed and executed. Also you need some smokes and flashes. If you don't have them you're doomed anyway. This strategy is just as effective as the players doing it are skilled. Stupid players will do it in a stupid way and likely fail. Good players are able to do it properly.
A successful B rush requires some aim and quick thinking. Since it's a rush people standing still will probably fail the whole thing. And a rush is obviously not over when you're on the site.
Smoke the entrance to the tunnel and the box on the site, so if someone hides there they're blind.
Optionally (but recommended) you can smoke the doors to disable any snipers lurking around in the middle.
Throw at least 2 flashes: one on the site and one over the wall to the middle.
Get in and spread out: check the camping spots first (car, crates,...). Be clear who goes where before you're on the site. The CTs aren't blind forever.
Be prepared for the retake. A lot of B rushes fail, because people are not prepared and people running off chasing the CTs.
- Keep some grenades to counter theirs.
- If you're getting flashed, throw one back.
- Avoid spots close to the wall or in the line of sight to the CT spawn. You'll likely get sniped.
- Watch the time. It takes 13 seconds to get to B from A long and 8 seconds to get there from short. You can easily step on their brakes when they eat your flash right when they walk around the corner.
- Don't get involved in duels. If you see someone, shoot and back off. Just guard the bomb.
- Guard the tunnels, if you don't have someone left behind guarding the middle.
Boom. You win.
If you want to practice B rushes, try it in eco rounds. If you're on eco you still need those grenades. If they are, be prepared for unusual tactics.
PS: If you do it with four people it can be useful, too. Someone distracting the guys on A and then covering your back from the middle can be very helpful. The CTs will have to deal with that guy as well as trying to get on B, which can be big bonus for you... The tunnel is covered and maybe one or two extra kills secure the round for you even before they get close.