u can use it immediately without cast time

Read the tooltip. It's very self explanatory.

Actually, there is a little more to crystal frags that is probably worth some explanation.

Traditionally, the rule of crystal frags was that you never hard cast it. In fact, it was a good way to get booted from Groupfinder as a "noob". By hard cast I mean go through the channel where you wave your hands around an cast it. Sorcs would typically use a spammable like Force Pulse or Elemental weapon, and when they saw the skill proc (that is where it turns red and makes a sound), they would cast it because it is instant and more damage. This is what Alcast is referring too. While its good advice for most people, that line of thought is no longer 100% valid.

Crystal frags has basically two versions. The hard cast, where you do the windup, which is a .8 second channel (it used to be longer I believe). It also has the proc cast, which is an instant cast, just like the vast majority of skills, which does more damage.

You now have the option on sorc to treat frags as your main spammable. It is a little clunky, but you can light attack weave the hard cast of frags without losing pace in terms of global cooldowns, so technically, there is no reason to run another spammable like FP or ele weapon. You still want to always prioritize the proc, because it does more damage. This method has Pros and Cons.

The main Pro is that you save a bar slot so you have room for a shield (defense) or execute (damage), or whatever you need there. Hard casting frags is comparable to other spammables in terms of damage.

The main Cons are that because the channel is .8 seconds, it takes more precise timing to weave in a light attack, as you really only have a .2 second window to cast your LA. Practically, a lot of players lose pace on their rotation when they try to do this and for lack of a better term, it just feels a little clunky. Also, you CANNOT bar swap off of a hard cast. So if you hard cast a frags, and you next skill you need to cast is on your opposite bar, you are going to lose pace on your rotation. Maybe not an issue on Static rotations, but if playing dynamically, it is one more thing to juggle.

Personally, I prefer to still use a traditional spammable and play it old school (coming from someone that mained a trial sorc for years). But gun to my head to maximize a parse, I do it the new way with hard cast frags as my spam. It has a higher ceiling but lower floor.

Edit: also, not sure this was answered. It has a chance to proc every time you cast a magic skill on the bar that it is slotted. It should be on your front bar as DPS. Once it procs, you can swap to your back bar and back again, and it will still be proc'ed. Sometimes that makes sense. If frags procs at the exact time your blockade comes off cooldown, you should swap, cast blockade, and make it your first cast when you get back to your front bar. Otherwise, a frag proc is pretty much always your first priority on what skill to cast. If a frag procs at the same time your front bar skill comes off cooldown, cast the frag first. That way, when you cast the next skill, it has a chance to reproc your frags.

Edit 2: If you dont know what proc means, its basically a term we use when some sort of event or "procedure" is triggered. If your monster helm goes off, we call it a proc. When frags goes to instant cast, we call it a proc.

From Wowpedia: Proc is a common term used primarily in game programming to refer to an event - a "procedure" - triggered under particular circumstances.