Hab, it all depends on the spread bet companys current book.
Example, If they are currently in equilibrium and their value of long bets meet their number of short bets then they are risk neutral. However should a large long position be staked on the company resulting in them having a naked open bet in either direction, then sometimes, but not always will a company hedge this bet and purchase/short the actual underlying stock thus nullyfying their naked exposure. This obviously in its self adds to the underlying stocks volatility and pricing movements.
The company must make their risk position neutral and very often this results in the purchase or sale of actual stock in order to mirror the current bets the company has open, otherwise they'd lose money especially in micro cap stocks where no other hedge technique exists. Options and futures for example, exist in larger cap stocks.
By positions via spread bets and cfds being naturally highly leveraged, this in itself adds a considerable amount of volatility to the underlying stock as its money being wagered on the stock that isnt really there, its leverage.