The technique I use is to warm the wax to soften it a little (this won't be required if you're using Meindl Sportwax) and apply it to the boot sparingly with my finger end. The heat from your finger helps the wax to spread and be absorbed into the leather (you could use a cloth if necessary).
I've heard this before....using your fingertip instead of a cloth.
I usually apply pre warmed wax with a soft cloth, then allow it to dry/sink in for a bit before buffing up with a brush.
Any 'rubbing' action, whether you use your finger or a cloth, will create heat...surely?
I think over waxing boots or applying wax too often has a detrimental effect on the leather. They don't need treating after every use.