Try as you may you can't actually stop a full load in transit. You can ruin the full experience and enjoyment of it, but its biology at that point. I am told however that there is a stage of excitement that it peaks and can quickly be stopped by several techniques http://ruinedorgasm.com/ But there...

When you bring an erect cock over the edge and don't touch it he'll bob up and down as it spasms. What about a soft one? Will it move a lot? I'd think it would start to get hard once over the edge. Does it feel as good as being hard? I'd think it'd be worse due to less full length stimulation, but ...