This talent was changed significantly in patch 2.0.
Previously, the tooltip reads "chance to return to stealth", but in practical use, "chance to remain in stealth" was the actual effect.
Core PvP specced rogues should be aware that since patch 2.0 all crowd control abilities (including Sap) last a maximum of 12 seconds in PvP, unlike pre-patch where Sap lasted longer.
In patch 2.1 the base skill Sap will not break stealth, Improved Sap will now then be renamed as Dirty Tricks and increases the range of Sap and Blind by 2/4 yards whilst decreasing the energy cost by 25/50%.
Energy cost reduction
Tactics / Tips
Sapping is dangerous against mobs that can detect stealth (these have a blue icon above their heads). You will usually not be able to sap these mobs. If they have non-detecting mobs with them, you may be able to use distract to do a sap, but the detectors will probably see you immediately after you do it.
Shadowstep, a 41-point talent in the Subtlety tree, can be used to get behind a stealth-aware target, provided that you can get in within twenty yards without the mob noticing you. Master of Deception helps here. Keep in mind that mobs can only detect stealth in a 180-degree radius in front of them.
If you are a dagger rogue, you should already be spending points on opportunity, so Dirty Tricks is truly an excellent 2 point investment and well worth it. Sword rogues may find it less tempting.
When a target is sapped he cannot give a combat flag to the rogue unless provoked otherwise by a quest event or by attacking that target's grouped allies, this makes gathering herbs & ores and quest items that are surrounded by humanoids much smoother as you can Sap, gather, and then re-stealth.
This talent is extremely useful against mages. It makes easier for you to blind them after a Frost Nova, or sap them when they're aware of your presence and start spamming Arcane Explosion (both AoE and sap will have 10 yard range, you can step forward between two explosions with enough skill ).