In straight and level flight, to remain at the same speed, lift must equal weight, and thrust must equal drag. If the engine power (which produces thrust) is reduced, then the drag will be greater than the thrust and the aircraft will begin to slow down.

If the aircraft slows down, the airspeed decreases, and therefore the lift from the wing would decrease, however, it must remain the same to maintain altitude. Therefore, the pilot (or autopilot) must pitch up to increase the angle of attack, this allows the wing to produce the same amount of lift with a lower airspeed. If this continues, however, the aircraft will eventually either stabilise at a lower speed with less drag, or if the thrust is too low for that, it will continue to decelerate until the angle of attack is too high, and the wing stalls.

This is more than enough to answer this question, however if you wish to look at the lift formula for an even more in-depth explanation of this, we will do this now.

Lift can be calculated as follows:
L= C_L × ½ rho × V² × S

Where:

• C_L is lift coefficient (without dimension)
• rho is density
• V is velocity [m/s] (velocity is TAS)
• S is surface area [m²]
• L is lift [N]

If we assume density (rho) and area (S) are constant, then that leaves us with lift being proportional to C_L x V². Therefore, any decrease in V will require an increase in C_L, and vice-versa, and C_L is directly set by the angle of attack (AoA) for a clean wing.