A check valve is installed in the upper section, which creates a high-pressure pocket of oil in the lower section of the lifter. As the cam lobe hits the lifter, it pushes it down and out of alignment with the oil supply galley, blocking oil from exiting the lifter.

A one-way check valve inside the lifter holds the pressure inside the lifter as the valve opens. Since oil is incompressible, the oil trapped under the plunger prevents the plunger from compressing and the lifter act like a solid lifter to push the valve open.

When the valve is closed the lifter is on the base circle of the cam and its cavity fills with oil. As the camshaft rotates and opens the valve, the piston is forced down and a check ball is usually employed to close the oil inlet orifice. As oil is considered incompressible, the piston cannot move because oil is trapped below it.

This traps oil in the check ball cavity and makes the plunger assembly move with the hydraulic valve lifter body, and then that moves the push rod and opens the hydraulic valve. The oil pressure inside the check ball cavity prevents the spring inside from compressing further.

"The hydraulic valve lifter or tappet consists of a body (cylinder), plunger, plunger spring and check valve assembly... Engine oil under pressure enters through an oil hole in the lifter body and flows into the space between the body and plunger. The oil pressure forces the plunger up against the push rod seat, and the check valve closes to trap oil in the lower chamber. This trapped oil forms a solid, incompressible column during cam lift so the lifter transmits motion to open the valve."

Hydraulic lifters eliminate the clatter and the need for periodic adjustments by maintaining zero clearance when the engine is running. They do ...

Hydraulic lifters operate by automatically adjusting to maintain zero valve clearance. Oil from the engine lubricating system flows through a small orifice and a one-way check valve into the pressure chamber in the lifter. During the cam’s base circle, the check valve is open and the plunger moves to take up lash. When the cam lobe rises and loads the lifter, the check valve closes, trapping the oil so that the plunger cannot move downward and the valve opens normally.

"Oil pressure from the engine's gallery pushes the internal check valve open, allowing oil to fill the internal chamber. This action effectively extends the plunger to eliminate any gap. As the camshaft rotates and lifts, the check valve closes, trapping the oil; because hydraulic fluid is incompressible, this creates a solid link to actuate the engine valve. These lifters automatically adjust for thermal expansion and wear, eliminating the need for manual valve adjustments."

"A check valve is a simple yet essential component in hydraulic systems, designed to allow fluid flow in one direction while completely blocking flow in the opposite direction. Because of this function, it is often referred to as a one-way directional control valve... A check valve installed in the inlet line prevents oil from draining out, ensuring the pump maintains its prime and is ready for operation when restarted."

"A check valve, also known as a non-return valve, is a one-way valve that allows fluid to flow in only one direction. When the pressure of the fluid exceeds the spring force in the valve, the valve opens, allowing fluid to pass through. When the pressure drops below the spring force, the valve closes, preventing backflow of fluid... Without a check valve, the fluid could flow back to the pump, reducing the pressure and causing the cylinder to retract."

The hydraulic lifter contains a check valve whose function is to admit oil into the pressure chamber while the cam is on the base circle and to prevent oil from flowing back when the lifter is loaded. Once the cam lobe loads the lifter, the check valve closes; the incompressible oil trapped under the plunger prevents it from collapsing, so cam lift is fully transmitted to the valve.

"Oil is allowed to go through that hole and then it matches another hole in the plunger and then that fills up the plunger with oil... and some of it will come down through the check valve and fill up this chamber down below the plunger. As soon as the lifter body tries to lift up... that will create a hydraulic lock and that's created by this check valve. All the oil that's built up in here, it's locked in there; that check valve closes off just the slightest bit of movement, so that all gets locked up. That lifter is now a solid lifter and it opens up your valve."

Once the lifter has oil in it, the check ball valve in the lifter should stop you from pushing the plunger down.

"Hydraulic lifter works by using oil pressure to keep the lifter touching the cam and the valve spring. Oil enters through a hole on the side, and fills the little cups inside. As the cam lobe hits the lifter, it pushes it down and out of alignment with the oil supply galley, blocking oil from exiting the lifter. This effectively creates a solid pocket of oil, which can't be compressed. A check valve is installed in the upper section, which creates a high pressure pocket of oil in the lower section of the lifter."

So under normal operation, the camshaft spins, pushing the lifter and pushrod up into the rocker arm, and the rocker arm pushes the valve open.