Thanks for the pictures Tony. Here's the explanation.
The hole for the pin is closer to the top of the guard than it is to the back. Using this information, you can take a hitch pin clip of the appropriate size and spread it open a bit. When the pin is in the vertical position, there is just enough tension in the clip to prevent it from falling out, and yet allow it to be pulled out with very minimal force.
When the clip is in the horizontal position (normal laying down resting position), it may only be removed with a significant dedicated amount of effort.
In the vast majority of positions for the light to be in, the clip will be in the horizontal position (i.e. laying down on the body of the light) and will be difficult to remove. Moving the clip to the vertical position, allows the clip to be very easily removed.
Anyone familiar with the securing clip on load leveling trailer hitches will recognize the principle.
One final mod I made to the clip was to cut off the unused part of the clip as shown below.
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