Quick answer
A hydraulic lowrider swaps the factory springs and shocks for hydraulic cylinders. An electric pump pushes fluid into those cylinders to lift a corner, and a dump valve lets the fluid back out to drop it. Switches inside the car control each corner, and a bank of batteries feeds the pumps.
Everybody has seen the clip: a '64 at a car show, front end bouncing higher every hit until the bumper is up near eye level. It looks like magic if you've never popped a trunk on one. It isn't. It's plumbing, wiring, and a lot of patience.
Where it started
California's Vehicle Code put a floor under how low a car could ride. Section 24008, on the books since the late 1950s, made it illegal to drive a car modified so that any part of the body sat lower than the bottom of the wheel rims. Cut your coils or pile sandbags in the trunk and you were asking for a ticket.
The fix came out of aircraft surplus. Builders in Southern California started pulling hydraulic pumps and valves from military planes and plumbing them into cars, so they could cruise low and raise the car back to legal height when they had to. Ron Aguirre's 1957 Corvette, the "X-Sonic," is usually named as one of the first street cars set up this way, around 1959. Within a decade it was standard equipment for anyone serious about the scene.
The parts, one at a time
Pumps. A pump is an electric motor bolted to a fluid reservoir and a pump head. Hit a switch, the motor spins, and fluid gets pushed down a hose to the cylinders. Most street setups run two pumps, one for the front and one for the rear. Hoppers often run more.
Cylinders. These take the place of the factory shocks or springs at each corner. Fluid goes in, the cylinder extends, that corner rises. Cylinder length decides how much lift you get, which is why hop builds use long cylinders up front.
Dumps. A dump is a valve that opens to let fluid back into the reservoir. Open it and the corner drops. Fast dumps are what make a car snap down hard enough to bounce.
Switches. The switch box sits within reach of the driver, sometimes on a cord so you can work the car from outside. A basic setup has front, back, and side-to-side. Bigger setups give every corner its own switch.
Batteries. This is the part that surprises people. Pump motors draw huge current for a second at a time, so builders wire several batteries in series to raise the voltage: 24, 36, 48 volts and up for hop cars. That's why so many lowrider trunks are basically a battery rack with a chrome display around it.
How the hop actually happens
A hop is timing. The driver hits the front switch to drive the nose up, and as the suspension tops out and starts to come back down, hits it again. Each push adds energy, the same way you pump your legs on a swing. More voltage spins the pump faster, which gets the cylinder out quicker, which means more height. Competition hoppers are measured with a stick at the front bumper, and the numbers at the big shows get very tall.
Hopping is also hard on a car. Frames crack, ball joints and A-arms bend, and rear ends move. A car built to hop usually has a reinforced, often fully wrapped frame, and builders will tell you plainly that a street car shouldn't be hopped like a competition car.
Street car or hopper?
Most lowriders on the boulevard are built to lay low, lift, and maybe do a three-wheel at a show. Their setups lean toward reliability and a clean trunk display. Hoppers and dancers are their own sport, with extra pumps, more batteries, and frames built for punishment. Neither is the "real" one. They come from the same history, the one we get into in The '64 Impala and How Lowrider Clubs Organized Themselves.
If you're thinking about a setup
Go see a shop that builds lowriders, not a general suspension shop. Ask to see their trunk work and their frame work, and talk to their customers at a show. Budget for the electrical side as seriously as the pumps, and plan on maintenance: fluid, fittings, hoses and battery care. And check your state's ride-height laws before you drive it low on the street.