Good question - negative feedback inverts the output 180 degrees and feeds it back to the input. That is the technical answer.

The phase difference between two signals determines the result. If the signals are exactly 180 out of phase they cancel each other completely. That is the principle used in noise-cancelling headphones. See this article (remove the hyphens) for how noise cancelling is used in TRS (balanced) cables - h-t-t-p-s-:-/-/-w-w-w-.-a-v-i-o-m-.-c-o-m-/blog/balanced-vs-unbalanced/

Balanced cables they send both an original and an inverted audio signal over two different wires. Any noise picked up along the cable length will be affect both wires the same way. At the receiving end one wire's signal is again inverted and the two signals combined. That results in the noise being cancelled out and a gain increase in the two audio signals that are now in phase.

When you combine signals that are only slightly out of phase portions in introduces addition harmonics and even inharmonics changing the timbre of the sound.

The effects of negative feedback can be subtle. Use a sound that with a lot of timbre (harmonics and inharmonics) - just a sine wave isn't going to give you much. Also try setting the Dry/Wet to maximum Wet. Changing the delay time will affect things to.

When I find a good example where you can hear the difference I'll post it.

CAVEAT: with Yamaha you never know what they are really doing internally. In a lot of cases a value range from minus to plus can mean the actual value is just an OFFSET to an actual value stored somewhere else. That is how it works with things like volume and level. 

There are several discussions on the net where different audiences (guitar players discussing effects, for example) are trying to learn what the various features of different delay solutions do.  I also see articles from various sources.  Not going to reproduce that here because I'd have to better curate to be helpful.

For perception - the audio signal fed to the effect can have an impact (how static or "pulsing" as the dry signal) or, probably more importantly, if you're using stereo or not.  Negative feedback is generally done in the stereo field so having true stereo (L and R monitor speakers or headphones) would be important to hearing the difference between polarities.

Stereo vs Mono? 

 Negative feedback is generally done in the stereo field so having true stereo (L and R monitor speakers or headphones) would be important to hearing the difference between polarities.

Yeah - I noticed that several of the delay effects have parms where you can choose things like that. The Cross Delay has an 'Input Select' parm with possible values: L, R, L&R, L Mode 2, R Mode 2.

I can't find any info/doc that discusses the various effects and what their parms means. If anyone has seen such info in manuals for older instruments please post the source. Often the older documentation has info like that that just isn't available anymore.