Bindings have two basic functions which are,
These are kind of 'two sides of the same coin'. The binding needs to be positioned on the ski correctly, side to side and fore/aft. The distance between the toe and heel needs to be correct. This distance is known as 'Forward Pressure', fine tuning of this is done by sliding the heel, generally on a track. Alpine ski boots have a 'DIN Sole' which is a standardized shape and material density, there is an adult and Jr version. This standardized sole allows all alpine bindings to work with all alpine boots (as long as its adult/adult or jr/jr). The toe piece is gripping the toe lug of the boot (and in some cases the upper radius of the boot itself) and the heel is cupping the heel lug with a slight pressure that pushes the toe lug firmly into the toe piece.
Retention is based on the little scale you see on the toe and heel of your binding. It's called a DIN scale and is based on Newton Meters of force, a measure of twisting force. The scale is based on bone density studies and is calculated using Ht, Wt, Boot Sole, Age and Skier Type, basicly the Ht Wt Age and Boot sole are used to estimate body morphology, skier type is used to determine retention. With retention what differs is the way each manufacturer goes about holding onto the toe/heel lugs and how much 'elastic travel' they use. More toe and heel wrap means a more solid connection to the ski, more elastic travel means the binding will travel with the boot and recenter if pressure is removed. The binding will be less likely to 'pre-release' (a miss used term) but release will be slower, and may not occur at all.
Here's the thing, bindings primarily release due to the initial shock of impact, elastic travel mutes this impact and transfer release to twisting force. This twisting force can shred ligaments long before enough force is ever developed.