The ISC hand ascender is called the Ultrasafe because it is virtually impossible for the rope to come free of the channel.
Milled from a solid block of 7075 aluminum, the ISC Ultrasafe hand ascender has a unique captive-eye pin secured through the housing which prevents the cam from moving beyond a certain point - it simply cannot release the rope from the channel.
The teeth on the ISC Ultrasafe cam are cone-shaped rather than spiked (unlike other hand ascenders) and this is far gentler on your rope. When tested under load the teeth on the ISC Ultrasafe tore the sheath from the rope at 11kN but the hand ascender was still operable.
These hand ascenders from ISC are a work of art - if you are looking for safety and durability, here it is! Available in left and right hand versions.
Download the VTIO SRT paper which shows various ascent systems incorporating this style of hand ascender.