The Victorinox 111mm and Wenger 120 and 130mm models and some Wenger 85mm and 65mm models contain a locking blade, each family using a completely different locking system!
A few other tools and models also have locking mechanisms.
Victorinox Locking Systems
The slide-lock mechanism was used in the original 111mm Victorinox models, it was patented in 1985 and introduced the following year. The liner-lock models were introduced in the mid '90s. For about twenty years Victorinox produced both slide and liner-lock models. In 2017 the slide-lock models were withdrawn and only liner-lock models remained.
See details here and in the individual model pages.
Slide-lock
Slide-lock models have a grey sliding button built into the scale of the knife. Under the button is a small spring-loaded metal shaft that can move backwards and forwards. When the blade is opened, a slot in the blade spring under the blade is exposed and the small spring under the button automatically pushes the shaft forward into the slot, preventing the blade spring from moving and therefore the blade from closing. To release the lock, the button is slid back and the metal shaft under the blade is pulled away, freeing the blade spring and allowing the blade to rotate and fold closed. See diagrams below.
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Also see this post from Multitool.org for more images of the internals of this mechanism.
An email from Victorinox regarding the strength of the slide-lock. Aug 2008:
"The blade releases via the slider on the rear shell of the handle. It is comfortable and easy to operate and as you mentioned prevents accidental closing. The bestsellers in this series are the Picnicker, Rucksack and the Outrider. The Workchamp XL model is aimed at knife collectors.
At the moment we are developing and improving the spring for the locking blade and slider system.
During testing of a material sample the stress/strain curve is a graphical representation of the relationship between stress, derived from measuring the load applied on the sample and strain derived from measuring the deformation of the sample i.e. elongation, compression, or distortion. Steel generally exhibits a very linear stress-strain relationship up to a well defined yield point. The linear portion of the curve is in the elastic region and the slope is the modulus of elasticity or Young`s modulus. A torque in physics, also called moment, is a vector that measures the tendency of a force to rotate an object about some axis(center). The magnitude of a torque is defined as the product of a force and the length of the lever arm(radius) M=FxI. To stay in the elastic region a torque of maximum 10.8Nm should not be exceeded.
We found the slide-locks to be insufficiently robust and sturdy for fire brigades, rescue services, police and army forces. For this reason we developed new models (nos. 0.83xx - 0.84xx) with the same shape and size, but much stronger, with thicker rivets and blades, a liner-lock for the blade and a powerful screwdriver. In this range you will also find the Victorinox one-handed opening lock-blade (e.g. in the Rescue Tool)."
Liner-lock
The liner-lock mechanism was the second locking system used in Victorinox Swiss Army Knives, introduced in 1993/4. From 2017 all Victorinox 111mm models are liner-locking.
In these models the liner between the main blade and openers is made of steel (not aluminium). The liner has a sprung flange that automatically springs out over the blade tang when the blade is opened, preventing it from closing. To release the lock the flange is pressed away from the tang and the blade can be closed - see PRESS in the images right. Care must be taken with this operation!
A nice attribute of this locking system is that there is another flange at the other end of the liner enabling the cap-lifter/screwdriver to also be locking. This is a welcomed addition and makes the tool very strong as both a screwdriver and a pry bar.
According to Victorinox the liner-lock mechanism is more robust and can handle greater stress or load before failure.
Liner-lock on the Main Blade
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Wenger Locking Systems
Liner Lock I
The Wenger 120mm Ranger series models were the first Swiss Army Knives to use a liner-lock mechanism. They featured a sliding lock release button on the side of the knife, very similar to the Victorinox slide-lock button, however the mechanism is completely different. When the button is pulled back a nub under the button engages with a nub in the liner which pulls on the liner to disengage the lock on the blade.
Liner Lock II
The models in the Wenger 130mm series (originally known as the New Ranger series) and 85 mm Titanium Series feature a liner-lock on lock the main-blade. Release of this lock is accomplished by depressing the white and red Wenger cross on the side of the handle. The logo serves as a lock-release button, and neatly pushes the liner to disengage the lock.
This same mechanism was carried forward into the Victorinox Delémont range.
Packlock
Wenger uses their Packlock system on some 85mm models. This system uses a notch on the backspring and a mating notch on the tang of the blade that interlock when the blade is opened. A lock-release button is positioned above the scale on the top of the knife, pressing the release button pushes on the backspring to disengage the lock. This lock is not as secure as the other mechanisms, and is more of a limited safety feature. It could fail after prolonged use and if the original machining was not precise. (Editors Note: References or test data for these statements required).
This same mechanism was carried forward into the Victorinox Delémont range.
Tool Interlock
A very few 65mm Wenger models, such as the Viceroy, feature a locking knife blade that is interlocked with the nail-file. To release the lock, the nail-file must be opened to the 90º position. See instruction booklet here.
Pressure Lock
The cleverly designed Wenger pressure-locking mechanism is used on most Wenger screwdrivers. With this system the lock is automatically engaged when pressure is applied downwards on the screwdriver as it is pushed against a screw. The pivot hole on these screwdrivers is slightly enlarged allowing the driver to move very slightly backwards into the knife frame, engaging a notch in the tang into a small recess in the screwdriver spring. The lock is automatically released when pressure is released.
This same mechanism was carried forward into the Victorinox Delémont range.
See this SOSAK/Swiss Army Knights article: Wenger Locking Screwdrivers - February 2nd, 2010