Basically the rhoades lifters bleed down below 3500 RPM and take about .025" lift and about 10° - 15° duration at idle... Then as the engine speeds up, they don't bleed down as much until you get to 3500 RPM and then you get the full lift and duration of your cam... They even work with high volume and high pressure oil pumps...
They give more vacuum at low RPM's, better throttle response, and increased vacuum...
When I put the MP 484/284 cam in my 340 cuda, with stock lifters it idled choppy at 1100 RPM and shaky 10.5" vacuum.... I installed rhoades lifters and I was able to get it to idle much smoother at 800 RPM with 13" vacuum....