This is a little known trick that I've been using to hide a ton of troops. Essentially darknests are an easy way to maximize the amount of troops you can hide because you're essentially sending them to players without popping fury. This comes in handy if you want to avoid getting into situations where you're unable to shield because of sending reinforcing players and this option helps alleviate that problem.
I will say that you are at the will of your own guild members to relinquish your troops after they have joined the rally, but that is a minor concern compared to them being killed. Watch the video if you want to know how you can actually increase it and all the ways I go about using it! Subscribe to my channel if you want more tutorials related to Lords mobile!