Do not let hammer form introduce you: on Jayce jungle the clean opener is usually cannon Q then E from fog, ranged W while they react, and only then the swap forward, because the champion feels much worse when he shows up as a fake diver instead of making the target answer the gate shot first
Learn the camp-hop route—hammer E Gromp, buffs, or Krugs and instantly Q onto the knocked-back camp to hop the wall—because that little geometry trick is part of real Jayce jungle routing, steals, and exits, not just a flashy extra
Treat level 4 with a second point in Q as the first honest checkpoint: before that, full Q-E poke is often too expensive, and recent data says Jayce looks much better when that second clear turns into a timely buy than when an early river flip delays the whole sequence
Swap when the target has already answered cannon form. In practice that usually means one of three things happened first: your gate-Q landed and they are chunked, they burned flash or dash and now have to run in a straight line, or a diver stepped too far forward and you can use hammer E to shove the fight back in your favor. If none of those is true, stay in cannon longer. The common mistake is swapping just because you are in range. On Jayce jungle, hammer form is much cleaner as the collect or peel step than as the opening handshake.
When Jayce swaps weapons he gains Move Speed for a short duration.
Hammer Stance: Leaps to an enemy dealing physical damage and slowing enemies.Cannon Stance: Fires an orb of electricity that detonates upon hitting an enemy (or reaching the end of its path) dealing physical damage to all enemies hit.
Hammer Stance: Passive: Restores Mana per strike. Active: Creates a field of lightning damaging nearby enemies for several seconds.Cannon Stance: Gains a burst of energy, increasing Attack Speed to maximum for several attacks.
Hammer Stance: Deals magic damage to an enemy and knocks them back a short distance.Cannon Stance: Deploys an Acceleration Gate increasing the Move Speed of all allied champions who pass through it. If Shock Blast is fired through the gate the missile speed, range, and damage will increase.
Hammer Stance: Transforms the Mercury Hammer into the Mercury Cannon gaining new abilities and increased range. The first attack in this form reduces the target's Armor and Magic Resist.Cannon Stance: Transforms the Mercury Cannon into the Mercury Hammer gaining new abilities and increasing Armor and Magic Resist. The first attack in this form deals additional magic damage.