you would probably get a ratio of new NSN shares, something like 1 NSN share for every 5 or 6 shares of Nokia, the price of nokia would go down like you would be recieving a dividend, the valuation of Nokia would adjust being subtracted by the value of the spin off company. Whatever that may be.
If NOK now is value at $17billion and if NSN is value at $5billion then after the IPO NOK will have value of $12billion if it stay at current price. And we would then have shares of NSN value at market $5billion but market can value it higher meaning price will then go up or they can value lower so it can go down so we don't know what NSN will really be value in the open market. But in any news of any spinoff price tend to go up, so if at current price then we should go up 20%+ until IPO of NSN.
One possibility is this. NOK shareholder will receive 1 share in NSN for every 10 shares of NOK they hold. Imaging NSN would IPO at $20/share, then NSN would have a value of $7.4B (just imaging). And, imaging that the Old NOK share traded at $7.00 before NSN IPO. On the IPO day, NOK would open at $5.00.
You would get 1 share of NSN for every share of Nokia you have or a ration that would be the same for everyone. For example 1 for 1 or maybe 1 share of Nokia for 10 of NSN.
Anyways Say Nokia was trading at 5.00 at the time. After the spinoff assuming a 1 for 1 share deal.... the results might be Nokia shares valued at 4 and NSN 1. Likely however it would be more like Nokia 4 and NSN 2. Then trading would continue from there.