These markers use a pressurized valve plunger to release air in the same way as mechanical markers, however in this case the valve is opened using air pressure instead of mechanical force. Generally speaking, these designs are considered to be further developed, despite some being quite old. This is likely due to the inherent recoil-reduction since these markers often offer low moving mass (which can influence recoil).
The exact firing methods differ from one-another very greatly. I tend to categorize them as either requiring a burst of force to fire, or by removing an "idle" force to fire. There are also hybrid combinations of the two.

Increasing force:

Nova (wmv)

Legion Shocker

These markers fire by adding force behind the poppet, which pushes it open and fires the ball. The firing design for the above markers is very similar, however the Nova uses an articulating barrel instead of a traditional bolt. In the Nova and Legion Shocker (as well as Mayhems) the dump chamber pressure holds the bolt/barrel in the closed position, which allows the bolt/barrel to move when the chamber is expelled.
Click for a slower Legion Shocker animation.
The Legion Shocker was a marker concept devleoped by SP in 2001-2002, but was never released. Please note this is not the same as the Russian Legion private label Shocker. All Shockers available for purchase function the same, using a spool valve.

Shocker Sport

BL Onyx

These markers function by adding force to one side of the poppet to release the seal and fire the ball. They don't necessarilly function similar, though. Onyx is a literal electropneumatic conversion of the mech design used in the A-5, and has other applications. Click for a slower Shocker Sport animation.

Decreasing force:

mQ valve

mQ valve

Deadlywind Aedes (mQ)

ICE Epic

WM Assault-80

The mQ is a poppet-based valve system that initiates a small leak in order to fire the valve (firing by means of removing pressure/force). First valve animation from Vantrepes. The Aedes marker is a stacked-tube design using an mQ valve to fire (click for a slower Aedes animation).
The ICE Epic uses a unique trap-door action to seal off the chamber, however the valve is fired by removing pressure from one side of the poppet in a similar method. The Epic is mechanical, however, so the removal of force is carried out by a mech trigger.
Lastly, the Assault-80 functions by pushing the poppet/bolt combo forward, then releasing pressure behind the poppet to actually initiate firing. The layout of this marker is similar to the Legion Shocker and Nova, however the valve action has been switched (decreasing instead of increasing). The Assault-80 is an open bolt version of the Mayhem, which is a redesign of the Nova that uses a bolt instead of an articulating barrel. Click for a slower Assault-80 animation.

Hybrid:

Invert Mini

King Cobra/Invert

Invert Mini

These are technically "decreasing force" but with a twist. They use a poppet to control the release of pressure, by simultaneously removing force from behind the poppet and increasing it on the front, thus moving the poppet and firing the marker. The bolt is pushed forward using pressure from behind the poppet.
The first animation (courtesy of Manike) is of the Invert Mini, based on the older King Cobra marker. The second animation (courtesy of Nadsenoj) is of the original King Cobra design. The two are operationally similar, except the Invert uses modified parts.
The third animation (of the Mini) shows the valve action with greater detail. Click for a slower animation.