On a single sided drive, there is a little pressure pad on the opposite side of the disk head that keeps the disk surface against the head.
On double-sided drives, the disk is sandwiched between both heads, which basically provides the function of what the pad did.
But when Apple was designing their first double-sided drive, they had the idea that directly opposing the disk heads, like everybody else did, would be bad somehow. So they did this instead.
This is the twiggy disk.
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@gloriouscow vendor lockin at it's finest, even back then
