Again, it was suggested to me to watch the g-meter, which was irrelevant and hence my flippant answer.
I'm very comfy with the simple stuff and the difficult. But this is simple, transitioning from equilibrium of 4 forces by lowering the nose with constant power will produce an acceleration in 2 axis - vertical and horizontal. If the desire is for a constant airspeed then either thrust has to be reduced (less power) or more drag added. Your lift vector is now pointing away from the vertical due to your descent angle (or glide angle) but weight remains the same. Down you go, with the lift vector sum less than the force of gravity acting on your weight, initially in in non-equilibrium / accelerated flight. If/when equilibrium is achieved in the descent with both the vertical and horizontal acceleration at zero the descent will continue as the weight vector remains in the same vertical plane but the total lift vector sum is reduced as it is angled towards the flight path, working (partly) in opposition to drag.