It's pages 8-10 in the DMG. A really good example of play that I make all of my new-new players read through at least once.
In that example, they have a spider drop down onto a character to make an attack (and they have said character make a grab at it to throw it off). The spider effectively ends it's turn in the same square as the character - which the rules (which I found using that search - thanks!) clearly state you can NOT do. So that's what's been giving me fits trying to figure out the proper mechanic.
As I recall, that example of play (in the 3.5 DMG) is essentially a modified copy of an example of play from either 1st or 2nd edition, and reflects an older style of "ad hoc DMing" that was once assumed to be understood as part of the game, but is no longer as common.
Back then, DMs felt quite free to describe giant spiders "dropping onto" PCs and attacking them without necessarily treating it as an attempt to grapple, and if the player said that his character's response would be to "grab it and throw it on the ground," the DM would similarly not feel obligated to consult the grappling rules to resolve the action; he'd just call for an attack roll and announce the action's success or failure based on the result of that roll.
If you came to the hobby later in its history, it's totally understandable why the 3.5 example of play is confusing to you, however. While nothing in the 3rd edition actually suggests DMs slavishly adhere to the rules as written, the much more comprehensive, internally consistent rules of that edition (as compared to its predecessors) do seem to have led many to believe that there
must be a printed rule somewhere in the book for any given situation, and that a "good DM"
must abide by those printed rules.
But really, I think the example of play is just trying to show that a good DM relies on common sense to make informed judgments, and doesn't slow the game down by consulting the rules on every fiddly issue.