Two things you can't do with the FLS mixer are
1. You cannot route a mixer track to itself or any mixer track that is routed to it. This means you can't create custom delays and other kinds of feedback loops
2. You cannot send audio before the fader to another mixer track besides the sends
I used to use Senderella a long time ago before FLS had any kind of bussing, but kind of forgot about it once FLS was improved. Recently though I wanted to create my own custom delays since you can create unique effects by adding distortion, eq, automation, etc and realized Senderella could help
Senderella is a free VST plugin available from KVR
The basic concept behind a custom delay is you create a feedback loop with a single tap delay in the middle. Each time the delayed audio goes through the loop, it is processed with eq, filters, distortion, etc, so that each delay is shaped how you like.
You put a Senderella that is set to 'Return' at the top of the mixer, and a Senderella set to 'Send' at the bottom of the mixer channel. Adjust the send volume to taste to prevent feedback. (During experimentation I recommend putting a Limiter below the lower Senderella to prevent runaway feedback)
You can create a single tap delay using the delay 2 and turning the feedback all the way down, as well as turing the dry signal all the way down.
Add some processing effects and away you go. I've attached an example
Pre-fader sends are sends which are not affected by the mixer track fader. To create one, simply place the send on the last mixer track, and the return on the first mixer track that you want to receive the audio. (The same concept as the feedback loop only you're sending to a different mixer track.
One thing to be aware of is that there is some latency introduced when using Senderella. It's ok for delays and other time based effects, but when doing parallel processing you will need to compensate for the latency. I haven't investigated what the latency value is and/or how to determine it. Also not sure of the internal bitrate of the audio.. i'm guessing 16bit perhaps.. so watch your levels