If you are trying to modify a Gottlieb add-a-ball to do this, you can tap into the "Total Play Meter" circuit, which gets pulsed once by the score motor during startup. Most GTB AABs don't have this meter, but the wiring for the circuit is there.

Total Play Meter Wiring

A sharp razor blade will be a good way to remove the section of paint on the backglass where you mount this credit reel.

You're asking only during game reset, correct? I'm guessing only in a situation where the game has tilted could this occur, since normally you'd play through all the Wows.

Add a motor switch that pulses the credit unit stepdown if not at zero. Since it will be pulsing the credit unit decrement directly, you need to have larger contacts. You will want to tie this into the reset relay to keep power to the relay until it has reached zero. The easiest way to do this is to add a relay. Credit unit reset relay. That only needs three switches - one functions as a hold and releases once credit unit is zero. Needs to be on a cam with multiple fast bumps, not a slow dwell/rise.

Second switch allows/prevents motor from decrementing Wows.

Third switch releases main reset relay if it's the only thing holding it up from being reset.

Switch on the credit unit senses zero.

Reset Relay->CURR->Motor->Credit unit stepdown.

I can, will, and have been wrong before, but I think that'd work for the stated purpose?

You could also switch it around and have the motor switch activate the credit unit reset relay and that could have an additional switch to pulse the credit unit stepdown. Really lots of ways to attack the same problem.

Add a switch (and a spacer or two) to any of the switch stacks that is on a cam with multiple short bumps. Doesn't matter which one - you'll need slightly longer machine screws to hold the additional little bit of hardware. Route the wires nicely, and remember to use a plug if routing into the head. Use a little piece of heat shrink to ensure the bottom tab of the added motor switch doesn't touch anything.

OK, I see. Well, there is an added ball relay, you'd really want to tie credit unit step-up to that relay vs. to the ball count unit as it moves up and down throughout the game. You'd have to otherwise add a suppression switch during game start to prevent the first five balls from adding to the unit.

The method(s) I've suggested are fully reversible, but...

And above all else... document! Make sure you modify the schematic that lives with the game so that if/when you sell it the next person won't have to chase wire all over the game to understand what you've done.

It would help to know which title(s) of game you want to do this to since logic varies a bit with each manufacturer and era.

If I understand your desire correctly (tally WOWs during the game, and reset to zero at the start of each game), then what Bingopodcast describes is essentially what you want to accomplish but I don't think it will require an extra relay. What you want to accomplish is already there in the form of the score reel reset circuitry logic:

A system to keep pulsing something until all items are a zero
A source for the pulses capable of energizing the step down solenoid.

HOWEVER, IF you have NO desire to step the wow count down individually during the loss of each ball, a simple full reset Step up counter is all that is needed and the reset pulse would come from some other source through the Coin or Start relay and the score motor switch.

If, instead, you want to use an up step/ down step style stepper unit, a bit more is required. Where the necessary additions would be are at the zero run out of the added credit (Wow Counter). You will need to parallel one of the score reel run out switches (if they are normally closed, make it one that opens at zero). This one keeps the reset process going until the WOW counter has made it to zero. And one to feed the score motor pulses to the step down solenoid itself. Some games/brands accomplish score reel reset pulses through switch stacks on a reset relay. Others get the pulses directly from a large contact on the score motor. If, as was pointed out, the contacts on the score motor are not large ones, then your best bet would be to add another set of contacts to the score reset relay (one that pulses 5 times for each score motor cycle when score reels are not at zero).

It has not been discussed if you plan to defeat any Add-A-Ball ball count step up or replay (special) function. If not, you could probably get away with simply slaving the step up function off of the same circuit. The existing circuit should already have a "step up hold until EOS has been satisfied" circuit. This will provide an adequately long "on" pulse. However, if you plan to remove the add a ball step up or credit step up function associated with the WOW feature, you will also need to provide a hold circuit (addition of a relay).

And, Bingopodcast just pointed out, you will need to add a switch to the reset or game start up function to "blind" the WOW count step up coil of any up step pulses that are sent AFTER score reels are reset but balls to play step-ups have not been completed at the start of a game. Since this is normally done following the score reel reset process on most games just before it serves the first ball.

Also, in case you are not aware, Williams made a 3 function step/up step/down unit in the era of Grand Prix for the bonus units. This unit has a normal step down function (Single step) but also has an added relay coil that can hold the ratchet open for a Full reset at the start of the game. Just in case you decide to do something a bit more exotic.

Not required, but easier to reverse, and see at a glance is incorrect.

Those Williams mechs would work well.