Mechanics:
The way the spell works is the following: you cast it on a target hero, and you receive the last spell he/she has used for a duration of 3/4/5 minutes. You cannot steal passive abilities, only active skills. If the skill has a secondary ability (like Puck’s Illusory Orb has Ethereal Jaunt), you will receive it as well. The duration is of Spell Steal should be taken into account. In case you manage to steal a powerful spell, you should attempt to make use of it before the duration ends. This means that, for example, with a powerful team fight ultimate (like Ravage, Black Hole) you might want to tell your team to force fights. With a powerful ganking tool you might tell your team to Smoke up and find a target, etc.
You receive a static copy of the spell. This means that provided you steal, for example, Requiem of Souls, your Requiem will have as many souls as the SF had at the moment you stole the spell, and you will not be able to gain (or lose) new souls. You receive the exact level of the skill the target has regardless of your own level. The only case where this rule doesn’t hold is when the target has Aghanim’s Scepter: you will not steal the upgraded version of the spell but the regular one. If you have an Aghanim’s of your own, on the other hand, you will always receive the upgraded version regardless of whether the target has an Aghanim’s or not.
It is possible to reset the cooldown of a spell you have stolen by stealing another spell and afterwards stealing the initial spell. If just re-steal the same spell the cooldown wouldn’t be reset (only the steal duration would be).
Finally, as we mentioned, every spell that you steal receives the casting time of Rubick (0.1 seconds) which makes certain slow spells much more practical. For example, when you steal Leshrac’s Split Earth, you will cast it almost instantly, much faster than Lesh who has a very long casting animation. Nevertheless, spells with extended casting times (like Assassinate, Teleportation and Requiem of Souls) keep the casting time extension.
Usage:
Spell Steal is quite straight forward to use mechanically – the range is very big and the spell even goes through BKB, so the only thing you need to be careful about is not to steal a spell you don’t want. Try to cast Spell Steal immediately after you see a hero casting a spell you would like to have. If you delay stealing the spells even for a second, you can give enough time to your target to cast another less useful one – most good players will do this intentionally to deny you the opportunity to steal. For example, a Tide will always try to cast Anchor Smash as fast as possible after Ravage. In order to be successful, you need to try to steal the Ravage even before the casting animation (cast backswing) is completely over. Another way to increase the chance of stealing is to try to disable an enemy with your Lift right after he casts an important spell. This will make sure that he won’t be able to cast another ability, and you will steal the desired spell.
The difficulty of using Spell Steal correctly isn’t mechanical, but it is rather tactical. You need to have a very clear idea what you need to steal (a priority list if there are a couple of good spells), and you need to be patient and to mind your positioning if you want to be successful. For instance, if you know your team needs you to steal RP from the enemy Magnus, it might be a good idea to stay outside of the fight until the Mag casts the RP. As a result, you wouldn’t be disabled or killed before you have the chance to steal the spell. After he casts RP, you can immediately steal it and enter the fight (ideally with a mobility item) to use the RP and your other spells.
Another important aspect is to try to always have a spell available. Provided your Spell Steal ability slot is empty, try to steal anything, just so you have something. Naturally, if you find something better, replace what you have. Consequently, when you are fighting, you can actually cast what you have, steal another spell and use it immediately afterward – in case you use two good spells like that, you can easily turn a fight. Imagine playing against a team with an Enigma and a Tidehunter. If you managed to steal Ravage in the previous fight and Enigma casts Black Hole, you can use Ravage to stop it, steal BH and throw it at your enemies.