Some countries will vary, but the US and Australia are pretty similar (like many countries, they follow the US in many rules), and since I know where the Aussie rules are, I'll cite those:
Customs (Prohibited Imports) Amendment Regulations 2011 (No. 4)
Also I realise there's a new version, feel free to update, I can't figure out the blasted site.
Anyway, section 16:
The scope of the definition remains unchanged, but clarifies the intention of
terms utilised to describe the opening methods of the
various knives with deployable blades, including:
(a) a knife that opens through the use of inertia and or centrifugal force.
This is intended to cover knives where the blade of
the folding knife may be fully opened with the flick or double-action
of the wrist. The requirement for some skill to release the blade into
a fully opened position utilising centrifugal or inertial force is not
intended to preclude a knife from being a classified as a single
handed opening knife.
(b) a knife that opens through the use of gravity. This is intended to cover
knives which may be opened by force of gravity. The
knife may be additionally controlled by a lever or button, but
typically, applying pressure to such a device and pointing the knife
downward will result in the knife’s blade releasing and locking into
So a knife that can be opened with one hand may well include multi-tools, since you can slide a blade out with a single hand, with a bit of effort, making it a 'hidden knife', even if it is a broad definition.
Note however, that in the US (and presumably other countries follow suit in the majority, but check your local laws), some knives are now allowed, that is, IF:
- the blade is no longer than 2.36 inches or 6cm in length
- the blade width is no more than 1/2 inch at its widest point
- the knife does not have a locking or fixed blade (multi-tool?)
- the knife does not have a moulded group.
So it kinda sounds like they made the laws to cover switchblades and the like, and as an unfortunate by-product, these useful little knives and multitools get written out as a result :(