here it is for you - Law 19 defintions
[LAWS]A player in touch may kick or knock the ball, but not hold it, provided it has not crossed the plane of the touchline. The plane of the touchline is the vertical space rising immediately above the touchline.[/LAWS]
http://www.irblaws.com/EN/laws/5/19/169/during-the-match/touch-and-lineout/definitions/#clause_169
so you are standing in touch, the ball is in the field of play and in the air, and coming toward you.
provided it hasn't yet crossed the plane you are allowed to knock or kick it.
(aside - I think this is a terrible Law)
so with your feet in touch
- if you reach in to field of play and knock it back -- play on
- if you reach in field of play and catch it -- touch
after those two clear cut examples it gets more comp;licated, with other scenarios we often discuss (leaping, knocking on etc)