Thrust = force.
Drag = force.
Thrust = drag, keep speed same. More thrust, get faster, less thrust get slower.
Power = thrust x speed.
Drag gets more as you go faster, less as you slow down.
So, you increase power, which increases thrust, and get faster. But as you get faster, thrust gets less for the same power, and drag increases. Eventually they balance out and you're at a constant speed again.
Similarly, reduce power, and it slows down until drag is the same as thrust again.
This of-course assumes that you keep trimming for level flight all the time, but might help.
G