Unless you are suspended in the air (as in the jumping kick which most people does not do today btw) power has 3 sources:
1. Downward power which traditionally be referred to as heavenly qi (天气) – for a simplified understanding we can think about it as gravity.
2. Upward power (trad, earth qi – 地气) which simplistically can be thought of as normal power in newtonian terms a.ka. ground power.
3. Circulating power (trad. human qi – 人气) which is generated from body movement mediating the first two powers and giving them specific direction but also adding the result from circulation/spiraling e.g. dan tian movement.
When these three forms of qi are moving naturally and unobstructed we enjoy health and and important prerequisite to express internal power.
Another vital prerequisite for internal power are the 6 harmonies. For power to be able to be expressed without getting stuck somewhere and be optimally generated, the 6 harmonies must be established and present. The result of this is whole body power (zheng ti jin) which in Sun style is the preferred word and synonymous with internal power. This whole body power have 8 basic expressions (peng, lü, ji, an, cai, lie, zhou, kao).
The characteristics of Peng (ward off) is like a boat pushing water to the side or like a rotating ball redirecting the oncoming force.
Lü (stroke or roll back) is smoothly following and redirecting where the redirection can be down as in the stroke in the lan zha yi movements after bao hu tui shan or sideways as the stroke in 4 powers pushing hands.
Ji (press) is a a kind of closing power used in setting up for a discharge or pull down etc. and unbalances the opponent.
The characteristics of An (push) is a discharging force expressed in e.g. lan zha yi, bao hu tui shan and lou xi au bu.