The latency compensated mode is to record a compromised live performance and to specifically preserve the compromised part as it happened live.
I'll try to explain that loaded statement!
Performance that uses the Reaper mixing board live. Using plugin fx and so forth. And the deal is that the computer/interface system in question does not have the ability to run that setup with low enough latency to not perceive any lag. So you make a compromise and play with a little lag. Trying to anticipate things and struggle through it. Because you're anticipating things and working your performance around playing with a lag live, now you decide the best option is to record the audio strictly as is. Lag and all.
Normally when you record, audio is nudged back into place to cancel out any latency in the system. It's assumed you want this by default and that's usually the case. Recording while monitoring live input with an audio interface's built in zero added latency monitor feature, for example.
Because YOU were compensating live, the best option is to preserve that rather than line up the audio as though there were no lag. So you want to record the performance as performed with latency and preserve it strictly as is (for good and bad). That's what turning off latency compensation is for.
As far as creating sends on the Reaper mixer...
It's supposed to mimic a typical mixing board. When you're familiar with that, you'll look for where those buttons are in Reaper real quick and off you go. If you haven't used a mixing board before in real life, maybe not as intuitive.
If you've done those connect the dots things when you were little, you've got this! It's all connections from one place to another.
PS. You could also choose to record the output of that track. Or you could instead use the freeze or render features to record the output from that track that way.