As the name suggests they are improvised and theres no pattern to the design/build - as above many are remote detonated, but they can use all kinds of trigger mechanisms... if they started using flails regularly the insurgents would just adjust the triggers for a slightly longer delay before detonation.
Back in 2005/6 the insurgents in iraq stared using an iIRbeam across the road to trigger the ied when the front wheels came and broke the beam, so the americans added a piece of a tyre attached to the front of the vehcile on a long pole, within a few weeks, the insurgents added a timer to the beam trigger.
The V shaped hull design originated from Africa, as the place is strewn with land mines, so yet again we brought out the v shaped hull designs, the insurgents start using shaped charge weapons mounted at shoulder height, that will go through almost any armour.
It does not matter what we do, the bad guys always find a way to defeat our counter measures, and usually in a very simple way.