The behaviour of the π-nucleon and πΔ (1238) systems produced diffractively in high energy πp and Kp collisions is analysed under the assumption of helicity conservation and parity rule ΔP = (−1) ΔJ . The two possibilities of s-channel and t-channel helicity conservation are considered. The well-known Treiman-Yang test is generalized for the case of s-channel helicity conservation. The πN and πΔ systems are analysed in partial waves in their rest frame, and the number of individual partial waves is found to be strongly reduced by the combined assumption of parity rule and helicity conservation. Tests for the combined assumption are discussed. They require polarization experiments for π nucleon but some deal with angular and decay distributions in the πΔ case.