Ready to have your minds blown? Sailboats do not want to go forward when under sail, they want to spin stern over bow.
Overlay a sailboat on a clock so the bow is pointing right to 3:00, the stern is at the 9:00 position. The mast is at high noon. Under sail, the boat would rather spin clockwise rather than go forward. The axis of rotation depends but usually it's somewhere between the waterline and the bottom of the keel around midship.
The sails are well above the axis of rotation AND the sails exert some force. Its the opposite of a boat under power where the thrust is almost directly behind the axis of rotation and level with it.
Thought experiment. Instead of sails, imagine there is a line tied somewhere on a mast. It could be low against the deck, or up by the head. Initially, as you pull the line the boat will want to stay in place yet the mast will follow the string,
The physics are the same in any orientation. If you lay the boat on a table and pull the line the boat will spin until the the central axis is directly behind the line, then drag. If you yank the line the boat will have enough momentum to spin completely around. Like a pinwheel.
The only thing preventing a sailboat under sail from spinning stern over bow is the buoyancy in front of the center of rotation. Gravity and the spinning force is countered by the water pushing UP. If we take our boat now floating and pull the line it will go forward and also attempt to submarine. The amount of submarining depends on how rapidly the line is pulled and how high the line is on the mast. If the line is pulled forward while boat is heeled it will go forward less, submarine less, and spin more, buoyancy is still up, but the forward thrust is not down to water anymore. The thrust is towards the horizen. The boat will spin like it did on a table.
A good sailboat will have the focused energy of the sail (the line tied to the mast) as low as possible to provide the least amount of leverage. The higher the line the more leverage there is to rotate. The lower the line, the more power that can be applied to the mast and have it converted to forward versus angular momentum.
One way to adjust the "thrust line" down the mast is to reef the main. This reduces the leverge on the mast, reduces the forward thrust, and also the surface area that tips the boat over, especially for a poorly designed surfaces. Same for jib but usually reefed jib does not lower leverage, A jib will provide most of the forward thrust so the jib is often the culprit for spinning forward. This is different from a true weather helm scenario which is really about rudder balance.
The best natural explanation is watching a maple seed fall. Should have called it maple seeding.
So if Dave says dont let it heel he's got to be saying the boat is some combination of over leveraged, overpowered or top heavy.
I sail a hunter 170 when its not hot and stormy and an rc sailboat when it is. I rarely sail with all sails up unless im packing rail meat.