Every unit has unique cast animations for each of their spells. The cast animation of a spell is enacted as soon as a target for the spell is chosen, if it requires one, or its hotkey is pressed, if it does not require a target. These animations are not bound to specific spells, but rather to the ability slots of each unit and hero. The length of the animations varies for each spell as each of them is unique and made for the spell which is usually in the slot the animation is bound to. Although "cast animation" only sounds like a visual part of the game, it does have a mechanical influence on the gameplay.
Every spell has a certain time frame the casting unit has to go through in order to successfully apply the effects of the spell. This time is commonly known as cast point or cast time. This cast time is set for each spell individually, and has nothing directly to do with the visual animation of the spells. However, since the animation starts playing immediately, it indicates the cast point of spells.
In almost every case, the length of cast animations exceeds the cast point of spells. Upon reaching the cast point, the animation does not stop immediately, but continues playing. For most spells, the unit keeps on standing still during this part of the animation, and is unable to act until the animation has completed, if it is not canceled. This is commonly known as cast backswing.
The part of the animation which occurs after cast point has been reached is called backswing animation, and during it the unit is unable to attack or use abilities. This part of the animation is purely a liability; it has no effects besides temporarily impeding the unit, and as such it is beneficial to cancel a long backswing animation, which will allow the unit to execute other commands sooner. The player can cancel the backswing by ordering the unit to stop, move or simply by using an item or casting another spell during the backswing. Disables, including silence, will cancel it as well. However, queued orders do not cancel it. If orders are queued, the next order will be executed after the backswing finishes.
Backswing is individual to units and their abilities, and is not necessarily the same length even for spells with the same total animation time. For example if the entire animation of a spell takes 1 second, and the spell's cast point is at 0.3, the backswing will take 0.7 seconds. However, a spell animation of the same length with a 0.6 cast point would only have 0.4 backswing. Some units (and/or specific abilities) have long backswing animations, and it is on these units that cancellation of backswing will be most beneficial. Cancelling a short backswing is difficult, and generally achieves minimal difference in outcome, so it is not generally done.
Unlike other spells, which go into backswing (if there is one) after reaching cast point, channeled abilities go into a channel after cast point. If the unit is interrupted before reaching the cast point, the spell does not start channeling and does not go into cooldown and use mana. Only after having reached the cast point will a channeled spell go on cooldown and draw mana. If a unit gets interrupted during the channeling, the channeling instantly stops. Channeled abilities necessarily do not have backswings, as the channel must follow the cast point where the backswing otherwise would.
Ability cast points and backswings
Item active abilities are always cast instantly once their hotkey is pressed or a target is chosen. They have a cast point of 0 and they cannot have a backswing.
Just like regular abilities, some item abilities do not interrupt the current spell animation upon using.
- Armlet of Mordiggian – Unholy Strength1
- Glimmer Cape – Glimmer2
- Mask of Madness – Berserk
- Observer and Sentry Wards – Switch Wards
- Phase Boots – Phase
- Radiance – Burn1
- Ring of Aquila – Aquila Aura1
- Shadow Amulet – Fade2
- Shadow Blade – Shadow Walk
- Shiva's Guard – Arctic Blast
- Silver Edge – Shadow Walk
1 Toggling these does not interrupt.
2 Does not interrupt when targeting allies either.