The training for observing the body is one of the training that all monks have to train.
The first way that the monk observed the activities of their body is by breathing. They Breathes in and out like a normal human being. However, as they breathe in, they will tell them self that I am currently breath in. the same concept with breathe out. Another training is the postures of their body. They will always tell themselves of what they did, whether they sit, walk, or stand. All these training and observation gives them awareness and full attention of their action. In addition, they see that the body and life as impermanent. This give the monk the consciousness. On the other hand, the training for observing the mind is quite simple. It basically means that the monk will tell themselves and be mindful of what they think, their feeling and emotions. This training creates the awareness that the mind exists. So that the monk can be mindful and have consciousness at all
time
The relationship of these training and the theory of five aggregate is connected with how the monks contemplate them and clinging them as mental objects. The train give them awareness and mindfulness. The theory of five aggregate have them a knowledge of perception. The result is the same, both the training and the theory of give aggregate help the monks become mindful and have consciousness. These can also help people, because karma is everywhere and every action that we complete has karma. So, be mindfulness can reduce the chance of the bad deeds which result in the bad karma.