Answer:
The underlying principle is operant conditioning. The horse is made to move until winded and it wants to stop. When it stops, it must tolerate being in the trainer's proximity. If it does not, then it is made to move again. Therefore, the horse (usually) figures out for itself that it will be more comfortable to stand still and accept the presence of the human than to become more tired by running in circles as an attempt to escape.
It is possible to use this method in an abusive manner if the trainer does not pay very close attention to the horse's physical state, and if the trainer does not "reward the slightest try" by backing off on the pressure for the horse to move.
It is possible for a horse to become injured by overwork, or it may attempt to climb out of the pen, or injure its legs on the hardware of the pen, if it is pressured to move with too much urgency.
The round penning technique is not really a "horse training" method so much as it is a "horse taming" method. There are a few horses that are so nervous and sensitive that the method may be ineffective.
The effectiveness of the method actually has nothing to do with "love." With this horse taming method, the horse is not motivated by receiving love from the trainer. The horse is motivated by receiving relief from pressure (being allowed to stand still to catch its breath). Eventually, most horses will learn that being near the trainer is a "safe" place to be. Outwardly, the resulting behavior may give the appearance of "love," but that would be an anthropomorphic interpretation.