What is thermal plasma in-flight oxidation technique?
Original paper where this method (and, I guess, the term itself) has been introduced was written in Japanese in 1982 [1]. In 1997 there was a great overview published in English by Toyonobu Yoshida, author of the original paper, with a chapter (2.3) devoted to this particular method: 


  Processes used to manufacture materials by injecting a solid, liquid, or gas into a thermal plasma to use its high enthalpy and reactivity are referred to as in-flight plasma processes (IFP processes). 


Due to the nature of the method which utilizes plasma jet with reactants generated via various torch-types, there is actually a flow of chemicals reacting while they "fly", of course, in the plasma area. 



Various plasma torches provide various times-of-flight, temperatures and velocities. For example, the direct current jet torch has the following characteristics:


  An argon plasma of about $\pu{80 kW}$ that has been generated in air reportedly produces a temperature of about $\pu{11600 K}$ and a velocity of about $\pu{410 m/s}$... The reaction time for reactants injected near the anode at high temperatures is about $\pu{400 \mu s}$ and the gaseous raw materials are 
  mainly used for the synthesis of fine powders... The reaction time of the reactants is estimated to be $\pu{1 ms}$. 


Bibliography


Yoshida, T.; Akashi, K. Tetsu-to-Hagane 1982, 68 (10), 1498–1502. DOI 10.2355/tetsutohagane1955.68.10_1498.
Ultra-fine particles: exploratory science and technology; Hayashi, C., Uyeda, R., Tasaki, A., Eds.; Materials science and process technology series; Noyes Publications: Westwood, N.J, 1997. ISBN 978-0-8155-1404-6.