In snow:

• Under load the tread lug sinks into  the snow
• The lug deforms, the edges dig in: "GRIP" with low slip
• Cutting through the snow: "SLIDING" with high slip

Even pressure on the ground compacts the snow

Tipped edges create pressure peaks, which dig deeper into the snow

Rubber slides against snow and snow against snow

On ice:

• The lug deforms as traction is built up: "GRIP"
• "SLIDING" creates frictional heat; the surface of the ice melts

Pressure peaks are created on the sipe edges, no lifting up of the edges leaving the contact patch

Sipes increase traction forces by squashing the film of water (windscreen wiper effect)