When you inflict a critical hit, your bypass will be increased to reflect your blow striking a particularly vulnerable location. You have a critical bypass of 150% by default - for example, if your bypass is normally +10, your critical hit will actually have a total of +15 bypass (i.e., 150% of +10).
Some weapons have a critical bypass bonus for one of their commands, coloured cyan (along with the AP cost) at the top of the combat table. Weapons with a critical bypass bonus will state how much bonus they give (+10% or +25%) within their description. This is added directly to the 150% base critical bypass, so for example a rapier gives +25% critical bypass to all stab attacks, meaning that your critical stab attacks will have 175% of your regular bypass bonus.
Each combat technique may also provide an additional critical bypass bonus, depending on the colour-coding of its damage bonus: +20%, +10%, +5% or +0%.
Finally, certain Expert talents may also provide a critical bypass bonus (this will be described in the help file for each talent, if appropriate). All of these bonuses are added together before being applied to your base bypass.