Like Sanchin, Kotikitae comes with a warning. Long term makiwara or knuckle pushup training can lead to weakened hands or arthritis. The short term consequence of overzealous training is bruising. Condition yourself gradually and regularly and inform your partners if their pounding is the wrong strength. For other exercises there are many methods.
Partners bow, face each other in the same fighting stance, offset to the side a bit.
Very little if any arm extension is required. Then turn hands to palm up, and pull the arms back to starting position, again rubbing. Try to keep your arm from tensing. You should be able to lift the front leg ad remain balanced. Then develop this feeling in the rubbing.