Spellcasters have a lot of different spells at their disposal. Spells which grant bonuses (buffs) are one of their more potent spell types. Chain casting is the process of recasting a buff as soon as the duration runs out so it is always active.
This is not something which every game allows for as a default status as it represents a powerful bonus. However, the spells and associated mechanics in Earthdawn Fourth Edition (ED4) were developed with this possibility in mind. This article is about recommendations for handling it in your game and how it is supported in the rules.
The basic premise is simple: pause every few minutes to recast a spell, keeping it active indefinitely. This is appealing from a player's perspective as they can save precious rounds doing something other than casting buffs when a fight breaks out. There are also reasons a GM may balk at this concept: re-rolling constantly can break the flow of action and pacing, it feels like a free bonus, and often comes off as abusive and cheesy.
Here is my suggestion for handling this in game: determine how many extra threads are woven each time, what they are for, and consider the spell active at all times. Only allow this for spells which have a duration in minutes, anything less simply isn't feasible. If there is a need to determine a specific Spellcasting result, roll it at the time it is needed (don't allow Karma or other benefits) and the result stands. It is important to not allow any abilities which have associated costs to be used, such as Karma, as they would have to be paid each time the spell is cast. For a task which is on the tedious side, throwing in extras every time will start to get costly.
This is going to give players access to considerably more bonuses at all times, however there are a number of other costs associated with this practice. First off, it consumes a spell matrix. Reattuning a spell matrix takes 10 minutes, which means most spells will have worn off by the time the buff has been replaced, let alone put back in the spell matrix to recast, then replace again, etc. Reattuning on the fly is an option, but a costly one - at least 2 Strain every recasting, with the possibility for even more. Whatever spell is being continually renewed, it is in a spell matrix.
As well, there are not as many spell matrices as talents available to spellcasters in ED4. This increases the reliance on spell matrix objects, which occupy threads. Until Shared Matrices, but Thirteenth Circle adepts are pretty intense. The end result is keeping a spell active is going to have an opportunity and Legend Point cost for the spellcaster.
Hypothetically, all of the spells could be recast over and over again until some magical number of successes is reached. The good news for this situation is all of the applicable spells increase the duration from extra successes, making it a moot endeavor. Also entirely hypothetical; I cannot imagine this actually happening at a table. Though definitely something which would be argued about.
Extra threads allow for some spells to be upgraded from rounds to minutes, widening the pool of available buffs. Some also increase the number of targets with a single casting. It is going to be up to each GM to decide if they will allow a single spell to be active on more than one target without a thread for the purpose. I can say the intention was to allow for a buff to be deployed in combat on multiple allies in a single turn while in combat. Chain casting can be a controversial topic.
The other effect of extra threads is they can improve the bonus itself. Since these are limited by tier/Circle, the increases which are allowed are intended to scale. It also means the version of the spell cast by an adept of the discipline versus an adept who learned it through Versatility will inherently be better.
Hopefully, this sheds some light on how this topic can be addressed and was specifically part of the spell mechanics development in ED4.