This paper conducts a novel study of the P-Q coupling characteristics of grid-forming converters (GFM) under current-limiting and internal voltage-limiting in large disturbance scenarios. Initially, a simplified large-signal model is developed using internal voltage magnitude E and phase angle θ as key state variables. This model elucidates the nonlinear coupling between P , Q, E and θ, and examines how current and voltage thresholds affect the existence and accessibility of stable equilibrium points. By analyzing the trajectory evolution of operating points in the (θ, E) plane, the study uncovers the mechanisms and factors causing transient loss of synchronism in GFMs during phase angle jumps. It further details how internal voltage and currentlimiting influence the dynamic critical phase angle for loss of synchronization under the coupling effects of the P-f and Q-E control loops. Additionally, the paper addresses the dual effects of small-signal based P-Q decoupling control under large disturbances: enhancing resilience to minor phase jumps while potentially extending transient recovery. Hardware-in-the-loop tests confirm the theoretical analysis.