In this paper an experiment for the reaction π+ p→∧ +π + K is proposed. The total energy in the centre of mass system is fixed at 1900MeV. The kinetic energy of the final K-meson is then in the range 0-90 MeV. We propose to observe the final state ∧-π resonance and the cusp at the ∑πK threshold. From the correlation between the resonance and the cusp and from the angular distribution of the resonance, the spin of Y1* and the ralative parities among Y1*, ∧ and ∑ may be determined. Based on the diagonal representation of T-matrix in the channel space, a general phenomenological description of the resonance effect, the near threshold effect and the final state interaction is developed, which takes into account especially the correlation of the cusp with a nearby resonance. The general theory is applied to the present experiment as a special case.