Head cover or towel under the armpits drill.

I said (wrote)

In my experience, the vast majority of mid to high h/caps have an out to in swing path through impact.
If you are one of these golfers and you try this drill, to stop the headcover from falling, you will pull the handle of the club further left through impact. This increases the amount of sidespin put on the ball resulting in a bigger slice/pull, depending on where the clubface is pointing at the time.

Just my opinion of course