Shadow spikes appear when climbers are below the summit.

Imagine the mountain sliced in two at the climber's level. The mass of rock beneath casts a long shadow. Looking along and through it makes it appear as the 'conventional' triangular mountain shadow with its point upwards.

What about the mountain slice above the climber? It too casts a long shadow tube just like the lower mountain. When the eye looks along it, the upper shadow also has a triangular shape - but pointing downwards. If the summit is to the right then there will be a stronger spike in that direction.

Alternatively, think of the mountain above as a rather weighty cloud casting anticrepuscular rays!

To see a substantial upper shadow spike the climber must be at some distance below the peak.  Jakub and his companions were at that point only about 250 m below the summit - hence the faint spike.