Open offer
Regulation & taxThe offer an acquirer must make to public shareholders on crossing the shareholding thresholds prescribed in the takeover regulations, or on acquiring control, at a price computed under those regulations, unless an exemption applies.
In plain terms
It is for a stated proportion of the shares rather than all of them, so it is not a floor under your whole holding. Where the computed price lands above the market the price tends to sit just under it and the daily range flattens; where it lands below, almost nobody tenders and the chart is unaffected.