The base fits the “gutter” exactly and wedges in it. It is stable enough to keep it upright. The whole package is stabilized by the boom’s weight and the pressure from the main sheet. Once the sheet pressure is on the legs cannot slide inward, only outward, but the gunwale prevents them from sliding outward. The crutch can’t slide forward or backward because it is wedged in the “gutter” (It is a MK1+S boat).
To prevent the boom from sliding backwards and slip from the goosneck, I usually yank the downhoul taut. This too creates a small down force on the crutch that would hold it in place should the main sheet come loose. And finally the weight of the boom tent and the fact that the tent is it is tensioned to the gunwale also keeps the crutch in place.