I understand that the hardwood can expand and contract with temperature and moisture changes, but I don't understand how a 3/4" expansion space is going to work in reality.

If the hardwood floor expanded 3/4" towards the wall and the underlayment or the rest of the house didn't also expand with it, wouldn't the nails be completely ripped out by the time it moved 3/4", or even 1/4" for that matter? I can't see the floor expanding that much with respect to the house and not resulting in a far greater problem than running out of room to expand.

What am I missing? I haven't found an explanation of how the 3/4" expansion actually works for a nailed floor. I can see it working on a floating laminate floor.

-Stuart

3/4" is far more than you need for any ordinary floor. By acclimating your flooring prior to install and maintaining proper room temp and humidity, a 1/4" expansion should be very sufficient.

In some cases , there may be unavoidable moisture issues where other precautions need to be taken. Totally depends on the home and situation

You are correct! A 3/4" expansion is overkill in most cases. The problem is most manufactures require it or something close so we are forced to do certain things that may not be necessary just to keep the manufactures warranty intact. In reality most floors that are nailed down mainly move in one direction, the tounge side. This is due to the way the fasteners are placed.