Quick question: What precisely does this notation mean? Just realized I'm not 100% sure what the \_ means and my Google fu is not strong enough.

      From: Alex Fink <[log in to unmask]>
 To: [log in to unmask] 
 Sent: Thursday, November 3, 2016 11:14 AM
 Subject: Re: Back to Phonology
On Thu, 3 Nov 2016 15:35:27 +0000, The Scribbler <[log in to unmask]> wrote:

>Thank you, everyone, for all the help. I've got a couple more questions based on my homework. :)
>Question 1. In doing further study on affricates, fricatives, etc., I ran into the idea that the implosive palato-alveolar affricate is not a thing, but I don't have any difficulty saying ja implosivized the same way I say ba. Am I imagining things? Is this safe to include in my conlang with an implosive series?

Up to you.  But what nature seems to say is this: they may well be possible, there are scattered reports of them, but they're unquestionably dispreferred.  Usually one finds that a language with palato-alveolar affricates and an implosive series will have a palatal implosive stop instead, or occasionally some other sort of substitute like /j?)/, or of course just a gap.  

I assume the acoustic motivation for this is that it's very typical for implosives to have _no_ flow of air either in or out when the stop closure is released, vs. pulmonic stops which have a burst of air escaping.  If there's no flow of air then it doesn't matter if you immediately put your tongue into position for a fricative: the noise of fricatives comes from the turbulence of air flowing through a narrowed passage in the mouth, so no flow, no noise.  Can you hear a difference between your implosive "ja" and [J\_<a]?

>Question 2. When it comes to minimal contrast pairs, I have allophones in complementary distribution for various stressed positions, but stress itself is phonemic, changing the meaning of the word. In a situation where a'shi means something different than ashi', would the allophone for stressed a be phonemically contrastive with the unstressed a or would they both fall under a in the phonemic inventory and be treated as allophones in the phonetic inventory?
>I was doing the latter and treating stressed allophones as just that, but considering there is a semantic difference when stress is applied to a syllable, does that make it a minimal contrast pair and mean I should consider them separate phonemes? That always confuses me a bit.

AFAICT you can still get away with analysing stress as contrastive, and the difference between e.g. stressed and unstressed /a/ as allophonic.  The minimal pair in your last paragraph would be a minimal pair *for the position of stress*, not for a contrast between segments.  Suprasegmental phonemic contrasts are a thing: is the source of your confusion that you are (implicitly) assuming that all phonemic contrasts have to be in segments?