Woops. My information about slashback was innaccurate. Here you go.
Performed with 4+HS+S. It uses 2% tension. You cannot perform slashback during the first 10 frames after rising from down. The timing for a successful slashback is about 2 frames. After a whiffed slashback, you cannot block for 30 frames. Even after a successful slashback, you cannot block for 15 frames. You can however still perform another slashback. If you whiff a slashback, you can still try with another slashback again, but your guardstun will then be 4 frames instead of 2.
On that note, slashback usually has a guardstun of 2 frames if you're on the ground, and 4 frames if you're in the air, regardless of attack level. You take no block damage and your guard gauge does not increase.