As the spearhead moves forward, infantry units following in the gap behind them form up on both sides of the line of advance to protect the flanks.
Surprising them out of the Ardennes, where the Allies believed that no armoured force could operate, the German spearhead quickly started running for the coast at Dunkirk.
By repeating this maneuver a defender can narrow the front of the spearhead until it no longer commands enough width for the following infantry to effectively move.
When the Germans tried the same tactic again in 1944 during the Battle of the Bulge, the US Army could quickly "pick the corners" in that fashion and brought the spearhead to a halt within a few days.
The tactics of the German Blitzkrieg breakthrough were to spearhead the attack with massed armour, sometimes a whole Panzer division of 240 tanks moving in "combat echelon", but was intimately supported by mechanized artillery and infantry formations.