Kundalini is an incredibly fun practice that I was introduced to by one of my teachers in teacher training. It can involve pretty quick movements, moving on the breath with strong inhales and exhales. The movements can build a lot of heat in the body and the practice can be a lot more of a core and strength workout than you might expect. The focus in a kundalini practice can also be on different chakras – or energy centers within the body, either focusing on specific ones or working through all of them depending on the intent of the class.
If I teach an all levels flow class I definitely incorporate some kundalini elements and depending on how gentle a gentle class it is, may include some aspects there as well.