RV: I'll give an example for its use... and others may have some in regards to the Chen Style Tai Ji which is it well known part of.
Let's say one wants to do the MCO... maybe because they want to deal with blockages or reclaim vitality. Whatever.
One method to help in the visualization is to use a 'pearl' and orbit the pearl... but how to get a pearl?
As a procurer to the MCO, one wants to gather sufficent Qi as it should never be done in a depleted state; Qi easily scatters, etc.
One method is to breath into the LDT, I prefer to bring in through the crown (heavenly Qi) and heels (Earthly Qi), several times and get the stove warmed up... then start to let the heavenly Qi descend to the front of the LDT and the earthly Qi to ascend to the back of the LDT... this will start the rotation and one can use their hands in a circular motion to help encourage their visualization if they want.
Let it rotate for a while and continue like this but then start to compress the energy (which starts off the size of a drum) to a smaller and smaller size... eventually refined to a pearl. Stop the pearl spinning, drop it into the Huiyin and start the MCO. After done, expand the pearl back...
There are probably hundreds of different practices one might be involved in where they use such techniques. This is just one. This comes from Jerry Johnson's Medical Qigong program.