Ciao Simo,

"ratchet" e "bypass" sono due concetti distinti.

Tanto per cominciare, le forbici o cesoie "ratchet" hanno uno speciale meccanismo, ti allego qui la spiegazione in EN:
"Let's first consider how "anvil" pruners work. Anvil pruners have 1 sharp blade and 1 flat, non-cutting blade, or "anvil." When you squeeze the handle, the sharp blade is brought down against the branch you're cutting, and the anvil holds that branch in place so that the cut can be executed. If the sharp blade becomes dull, it will crush the stem instead of cutting it, so you need to replace blades when they get dull. With “ratchet” anvil pruning shears you can easily cut thick tree branches that regular cutters would stand no chance against. The ratcheting mechanism's settings multiply your hand's power, so that you don’t have to do the work – the pruning shears do it for you. As you cut through a branch, the ratcheting mechanism clicks and moves to the next setting, providing ever-increasing pressure as you cut. Ratchet-action pruning shears are thus obviously ideal for those with hand problems."
http://landscaping.about.com/cs/lazylandscaping/gr/ratchet_p...

In IT, si parla di "forbici a cricchetto", vedi per esempio questo catalogo di un'azienda italiana: http://www.ingegnoli.it/forbice-a-cricchetto-con-kit-ricambi...

Per quanto riguarda "bypass" si tratta di un meccanismo in cui le lame, chiudendosi, scorrono l'una lungo l'altra, come nelle normali forbici da cucina, per intenderci. Ti allego la spiegazione in EN:
"Bypass pruners usually work exactly like a pair of scissors, with two blades "passing by" each other to make the cut. At least one of the blades will be curved: a convex upper blade with either a concave or straight lower one. Some bypass designs have only one blade, the lower jaw being broad (like an anvil) but passing the upper jaw."
http://en.wikipedia.org/wiki/Pruning_shears

In IT si parla semplicemente di "forbici". Eventualmente, per distinguerle da quelle "con taglio a battente" (anvil), potresti usare cesoie vs. forbici.

HTH!