This is in response to the just released Chronotech supplement. It's the alternative (house) rules for Chronotech that my games run under. These aren't intended to supplant the existing rules, merely provide an alternative for those so inclined.
The future, the past, and the present are all probabilistic wave functions up through the macro-scale. “You”, at any given moment, are a single point in this space. From your perspective, both the future and past are ever expanding probability fields. Thus, there isn't a single future, but instead an infinite smear of possibilities.
When using Chronotech, you are continually receiving a low-level buzz of information from every possible future that would be sending anything back. Filtering this buzz out to glean important information is the challenge.
Information received and later sent by you is in the form of closed loops, which means that you're in the middle of a very powerful positive feedback loop. If you aren't careful, you can render this loop worse than useless.
These properties create some nasty trade offs that any practitioner of Chronotech must face.
Precision vs. Accuracy
Filtering down to a clear message means throwing away possible information from unlikely timelines. Unfortunately, the more you filter, the more likely the timeline you are focusing in towards becomes, regardless of whether its desired or not (i.e. you're making a self fulfilling prophecy by over-filtering your incoming data). Maximally filtered Chronotech (sending back only one clear message which is guaranteed to be sent in your future) can snap to any possible valid timeline that is self-consistent, regardless of your desires. You have one future, and it was chosen at random.
On the other hand, unfiltered Chronotech gives you vast swaths of overlapping and hard to understand information about all possible futures. It's relatively easy to find the 'best' possible future timelines with so many to choose from, but all information about how to achieve it has been rendered extremely vague.
Utility vs. Use
When using Chronotech, you personally are in the middle of the achronal feedback loop, and your behavior influences its utility to you.
If you are the sort of person who always obeys the advice of your future self, the futures in which you try other things will become low-probability, and thus you will gain no information from them. Alternatively, if you always do your own thing regardless of your future advice, your future probability space will be very broad, and thus the advice your future self-possibilities can give will be much more useful (since you will likely try every possible path).
As a result, the more you listen to and use your Chronotech, the less reliable it is, and vice versa.
GM's should keep in mind that the probability space is based on the current event, not over all time. Thus, if a PC only obeys their Chronotech in emergencies, it will only be inaccurate in emergencies.
The above rules encompass the “Interaction Permits Alteration” rules. The remaining Chronotech rules are still in full effect. Chronotech in a universe with a Stochastic Timeline should be slightly higher bandwidth, to compensate for its counter-intuitive limitations.
Note that while Chronotech 6 is sufficient to send a full digital mind back in time, the receiver will be accepting the overlapping probability space of that future mind. Filtering this down to a single consciousness brings up unfortunate ethical implications.