the key is that versatility from multiple attacks.
Volcano can be blocked. Hence, sometimes it will cause fbr lock on characters; it interrupts their teleport due to blocking, then, if it hits them, it has a % chance to put them into FHR.
FHR really works weird in d2- depending on what % of the maximum HP of a target an attack takes away, it has a % chance to put them into FHR. Theres no 100% fhr magic number like 1/12, rather its like if volcano takes away 1/16 of their hp, it has a small chance, 1/8 of their hp, a large chance, 1/4 of their hp, a 100% chance. Don't remember the exact numbers.
Volcano shoots a projectile every 2 frames. The projectile hits once at the location of the volcano itself, and hits whatevers on top of it, and then hits again when it 'lands' in a random aoe.
However, its hampered by nextdelay; each hit of volcano says you can't get hit again for X frames.
The initial explosion on volcano, getting hit by literally the volcano popping out of the ground, gives you 10 frame next delay
Further hits, after those 10 frames, have 5 frame next delay
So volcano will hit like this:
frame 0
frame 10
frame 16
frame 22
frame 28
since volcano only shoot on even frames, so even though the next delay is over on frame 15, the next projectile is at 16 (the one at 14 does not affect it).
Blocking, however, gets a free 10 frames since 1.10; after you block once, you don't need to use the animation again for 10 frames, on all characters
This is so that you can't get thrown in 'FBR lock'. So lets put together a thingy:
So lets say a sorc has 8 frame FHR & 6 frame FBR, and 8 frame FHR
Lets say she has a 50% chance to be thrown into FHR, and a 75% chance to be thrown into FBR
Frame 0 hits her, and she gets DOESN'T get put into FHR
She doesn't get interrupted, and now she has 10 frames to cast a teleport spell to escape. Since she teleports at 8 FCR, she can escape.
Now, if frame 0 hits her, and DOES throw her into FHR, she'll be in fhr till frame 8
at frame 8 she starts casting, but gets struck again at frame 10, etc etc
Same dice with her FBR
Long story short, it depends on a million factors; your opponents FCR, FBR, FHR, %block, the % of their HP you deal as damage, and more than anything, luck.
If you deal enough damage to throw her into FHR 100% of the time, she literally cannot escape via teleport; she'll keep getting thrown into FHR and this will interrupt her teleports
However, you'd need to deal like 1/4 of her hp to do this, so she'd die pretty fast anyway.
But the real key here is that volcano is so luckbased, that its fairly reliable to assume that it will interrupt an opponents casting as you use it; even with 25% FHR and against 75% blocking, thats still something like an 82%+ chance to interrupt someones teleport when you cast it on them. This is especially important, because it means that you can follow up your volcano with a namelocked teleport for a bear smack, which is the #1 shaman combo (volcbear). Since volcano naturally chainlocks for you, this is incredibly awesome.