Just in case people reading this thread don't know, here is the
basic info about it. Essentially, in early 1940 the original Nazi war plans accidentally fell into Belgian hands. ...
This is one of the reasons why reading Mays, or Horne is important. 'Original Plan' in the context of plan yellow had long been discarded. In October 1939 Halder insisted the current plans be discarded & replace ASAP. To that end he directed a series of tests as map exercises starting 7 November to examine a wide set of options. Specifically for the 7th November exercise he ordered three concepts tested. One was the schwerpunkt or mass of maneuver on the Belgian plain, second was with the Schwehrpunckt in the Ardennes, third with no specific schwehrpunckt. Rather a large reserve held back from a general advance and committed as the battle developed. All three plans failed in this initial test. Note that this was the origin of the 'MansteinPlan'. As Chief of staff for Army Group A he was responsible for detailing the actions of AG A in all three scenarios. In the Ardennes schwehrpuckt option he placed the single Panzer Corps available to AGA in the lead and crossing the Meause River in advance of the infantry. A few weeks later he received his expected promotion and appointment to command a infantry corps & his role in planning ceased.
Halder & Hitler were both unhappy with the poor prognostications and Halder kept the OKW staff, and the Army Groups, Armies, and Corps staff busy through the winter refining and testing multiple plans. These test exercises were both on the maps or game table, and in the field as part of the frequent training events that winter and spring. As the armored force increased to ten Panzer divisions and five motor rifle divisions the variations of the plans leaned increasingly on a massed armored group in accordance with Guderians ideas, supported by his followers and allies such as Kliest. While none of the repeated exercises resulted in a decisive or strategic victory those featuring the Ardennes schwehrpunckt were more likely to result in a tactical victory. Seizing on that thin result both Halder and Hilter put the majority of their attention to developing that option. The Mechelen Incident did turn Hitlers attention away from other plans to the Ardennes, but it had been on the table since the directive for the 7th November war-game. March 1940 seems to be the moment Hitler finally committed to the Ardennes option, tho he continued to waffle right up to late May after the battle started. I'm unclear on when Halder came to favor it over the other plans. It may have been as early as January, or as late as March.
Of all the German commanders only a few like Kliest and Guderian developed any real confidence in the Sicklecut plan. For those like Halder, Rundsteadt and most others it was more of a act of desperation, they could not see any better option. Mostly they saw a risky gamble that could very well fail, but all the other ideas tested failed worse. Hitler contrary to myth was not firmly wedded to any specific plan. He kept wavering from one idea to another, loathe to be trapped in a risky venture that had a very unpredictable outcome. After the attack started Hitler kept having crisis of confidence and tried to talk Halder and Rundsteadt into altering the plan on the 12th, 14th, 16th, and 18th May. He was described as increasing fearful the armored corps would be destroyed by Allied counter attacks.