Selecting 1st gear at speed

[SuperchargedVR6]

Whilst watching some youtube clips recently, sTiff Needle was driving a 911 through some S bends and on one hairpin bend he said: "I need 1st gear for this one". He must have been doing about 50-60mph on corner entry. How does 1st gear physically engage at that speed, and why don't the rear wheels lock up when the clutch re-engages?

In nearly every FWD road car (and some RWD) I've driven, 1st gear physically won't go in if the car is moving, even at 5-10mph! I need schooling on gearboxes I think!