I did a Hochkurz mash with a 30 min rest at 145 and a 45 minute rest at 160. While in the 145 range I did two iodine tests at 15 then 30 min, both were positive. It could be that the lower temp won't convert as fast as a range where both Beta/Alpha enzymes are working together better but I thought I'd share. Next time I may try a single rest around 153 and see what happens.
Wob, in your case, the mash jelled up when it was boiled during the decoction. Enzymatic action slows way down when mash jells. So, what you experienced is natural. Maybe, instead of going to 160F, go 155F (Alpha II) for 10 minutes and 162F (Alpha I) for 40 minutes. Try to beat the snot out of beta, after gelatinization. Then, work over the rest of the jell with Alpha.
The other brewers are not using a method that breaks down the hard chunks of starch. The decoction method works the starch over, real well. You should notice far less chunks left in the mash, if the mash is being boiled long enough.
The chunks of starch, as the picture on the previous page shows, are heat resistant and a single infusion saccharification rest does very little to the starch. The hard starch contains the majority of amylopectin. After the lautertun is drained, the chunks will still be in the mash, untouched by enzymes.
Since, nothing in a single saccharification rest does much with the chunks, Iodine won't pick up the starch, because it wasn't really in solution. But, it is still there, so when a brewer employs the mash out procedure, the hard starch bursts, due to the mash temperature being above the temperature at which starch bursts. Enzymes denature due to mash out temperatures and the amylopectin ends up in the bottle, unconverted.
When the hard, heat resistant, starch is ground to powder, enzymes will work at a snails pace on it, converting the starch to amylopectin. During a mash lasting five minutes, very little amylopectin will be formed. Mainly, being formed from amylose.
Mash temperatures in the Alpha II and Alpha I temperature range form non-fermentable sugar, which adds sweetness to the final product. Sweetness isn't the same as body and mouthfeel. Limit dextrin is responsible for body and mouthfeel. Although, limit dextrin is non-fermentable, it is not to be confused, with non-fermentable sugar produced during high mashing temperature.