Squig Hoppers are Night Goblins riding, or rather hanging on to, Squigs. They are included in Squig units and can fight in them too. The rider never fights, he is too busy trying to stay on, so a squig Hopper fights exactly like a squig. The advantage of a Squig Hopper is that he can deliberately bounce out of the unit at the start of the player's Movement phase so long as the unit is not already fighting in close combat, has declared a charge, or is subject to a compulsory movement rule such as that caused by a failed Animosity test, etc.
At the start of the Movement phase, as part of compulsory movement, the player indicates which of his Squig Hoppers will bounce out of their unit. Taking each Hopper one at a time, indicate the direction he will move in and roll 2D6 - the Squig Hopper moves exactly the distance rolled. However, on the roll of any double the rider is thrown immediately and the Squig Hopper becomes a loose Squig. If you have a spare Squig model, replace the Hopper with that. Otherwise just make a note and roll a Scatter dice to establish the direction bounced as described for a loose Squig.
If a Squig Hopper or loose Squig bounces onto any unit from either side it will attack immediately. Work out if the Squig hits and remove any casualties caused. The enemy cannot fight back. Then bounce the Squig again in exactly the same way. Continue to bounce each Squig until it lands on an empty space. If you're lucky it will bounce from one enemy unit to another, killing as many enemy as possible. If you're lucky...
If the Squig Hopper lands on a unit engaged in close combat allocate hits as you would for a template.
Once a Squig Hopper bounces out of his unit he is no longer part of it. He immediately counts as a casualty for purposes of calculating the proportion of the unit remaining on the battlefield, e.g. when determining if the unit can rally (ever the optimist, eh!).