At normal planned ejection speeds, the idea was that the LCC burned the perspex down to something like 1/32" thickness, and the airflow "should" open it up like a pair of doors to let you through. The knives were there to smash your way through if it failed.
The aircrew manual did say something like "there is a significant likelihood of serious injury to the front seat occupant from canopy shards following any ejection." Glad I never tried it!!