They leak down a lot with low rpm which keeps the valve from opening as far. At high rpm it pumps up more and lift is restored. Here'* some info I dug up:
"What if we could combine the low end of an advanced cam with the upper end power of a wider lobe cam? Here is a prediction of the Wolverine 5059 with Rhoads variable lifters and the 5059 with conventional lifters. Rhoads fast leakdown lifters shorten the duration and reduces the lift at low RPM. They also advance the intake lobe. All three parameters improve low RPM power. With Rhoads lifters, the valves open at the same time, but the spring pressure causes the lifters to leakdown quickly at low RPM. The valve will never lift as high, and will set down earlier. Rhoads claims a 10 degree reduction in duration, and a 10% reduction of lift, at low RPM. At higher engine speeds, less time is available to leak, and this effect is minimized. At about 3500, depending on spring pressures, lobe design, and oil viscosity, they begin to operate similar to standard lifters. Their characteristic clicking is caused by the earlier closing prior to the smoothing effect of the cam closing ramps. The plot shown reflects the maximum leakage for Rhoads, and the other reflects leakage of standard lifters. Obviously, neither plot is totally accurate, but it does show why the quick leakdown lifters, help the low end power. Should you use Rhoads lifters? If your cam is less than 225 degrees intake, you probably don't need them. If it is 230 or over and you are running an automatic, Rhoads lifters will definitely make your car easier to drive, idle better, and run quicker at the strip (providing you do not have serious traction problems)."
"Installed Rhoads variable (leakdown) lifters on the 041 with 1.65:1 rockers. Quarter-mile performance improved by .15 seconds and 1.5 mph over the 068 with 1.65:1 rockers. Idle quality was excellent, with high vacuum when the engine warmed up. Rhoads lifters cause a ticking noise similar to solid lifters.
Installed another brand of leakdown lifters. Car slowed down .15 seconds and 1.5 mph and would not idle properly, but the ticking noise disappeared.
Reinstalled Rhoads lifters and regained the lost performance (and the ticking).
The 041 cam, 1.65:1 rockers, and Rhoads variable lifters is the combination I've used over the past several years, and I used it during earlier testing of ported heads. In all cases, the Pontiac cams used for the controlled testing were the original Pontiac design, and not computer-enhanced versions."