If you are saying that he wanted to increase engine rpm, then why would he be deliberately lowering throttle?
It is possible to move the throttle toward closed, and increase RPM by moving the prop control to the "increase" position, which will be forward. The word "increase" on the throttle quadrant is in the forward control direction, which is the fine pitch position. Though nearly no WW2 fighters were "certified" in the context of today's plane, were they to be, they would have to comply with the following design requirement:
Sec. 23.779
Motion and effect of cockpit controls.
Cockpit controls must be designed so that they operate in accordance with the following movement and actuation:
........
(b) Powerplant and auxiliary controls:
.....Propellers----------------------------------- Forward to increase rpm.
Perhaps the pilot wanted to slow down very quickly to assist in positioning the plane for turn he wanted to achieve (not to overshoot the turn) and used fine pitch to create drag to slow the plane down. I do this gently in one of my planes on short final, when I want to loose the last few knots and settle into the flare. If I want to glide that plane, I select full "decrease" for drag reduction. I don't "modulate" a propeller control, it invites engine or propeller damage. Some planes (Cessna Caravan, for example) are set up for the propeller to move into a very high drag, flat blade angle when the power is selected to near idle, to aid a more steep approach. Reverse may be used on the surface.
Fine pitch may be selected for more thrust when combined with higher power, or fine pitch may be combined with very low power to create drag for slowing down. A more coarse pitch will be selected for extracting optimum power from the engine in cruising flight or otherwise for more efficiency.
It is possible to overspeed some engines by moving the propeller control toward "increase too quickly, even at low power - and doing so creates noticeable drag. It's a horrible thing to do to a geared engine.
As for WW2 pilot combat techniques, well, I'm not a WW2, nor combat pilot, so I'll step back from the discussion.