When is best to test the 'spike'?

Testing every 10-15 mins may give you a rough idea when that particular food, in that particular portion size, hits your bloodstream.
But RH doesn’t automatically follow a spike.
Your insulin response may be on time and sufficient to prevent a spike, but you may still experience an overproduction of insulin, which drives your bg down into a hypo.

it is easy to say ‘we are all different’, but it is sooooo true.
And then of course we have to add in unique food and portion sizes, stress and exercise levels. They all factor in.

Maybe you are overproducing insulin to prevent the spike from happening?
Obviously this is me speculating

In my case, years of RH (over 40 of them), I speculate that my own RH is likely to be a failure in the insulin ‘off switch’ which involves another hormone called glucagon, which acts as an insulin antagonist. I do get a spike though.

There are so many different ways that RH expresses itself, that I am sure there are many different things that lead to Reactive hypos.

In my case, during my last eOGTT, I only spiked just over 8mmols, which was unbelievable and not possible a before diagnosis, I still went hypo though.
RH used to be named idiopathic postprandial Hypoglycaemia because the research couldn't find out why even with a small spike we still went hypo!
Again, I believe that once you have any spike over your current normal levels, the trigger starts the insulin, usually a normal second response would stop when enough insulin is sufficient for the glucose derived. But because the initial insulin response is poor, the secondary insulin response doesn't stop because it had to when in my case insulin resistance and hyperinsulinaemia and high glucose levels were a norm due to eating carbs regularly enough to keep my blood glucose levels fluctuating all day!
There is a good explanation on Wikipedia, but this is generalised!
But the science is there!

Stay safe