In Universe Tankships v ITF  1 AC 366, at 383 per Lord Diplock:
The rationale is that his apparent consent was induced by pressure exercised upon him by that other party which the law does not regard as legitimate, with the consequence that the consent is treated in law as revocable unless approbated either expressly or by implication after the illegitimate pressure has ceased to operate on his mind. It is a rationale similar to that which underlies the avoidability of contracts entered into and the recovery of money exacted under colour of office, or under undue influence or in consequence of threats of physical duress.
In Progress Bulk Carriers Ltd v Tube City IMS LLC  EWHC 273 (Comm) at paras 36 and 44 per Cooke J:
36. … It is, in my judgement, clear from the authorities that "illegitimate pressure" can be constituted by conduct which is not in itself unlawful, although it will be an unusual case where that is so, particularly in the commercial context. It is also clear that a past unlawful act, as well as a threat of a future unlawful act can, in appropriate circumstances, amount to "illegitimate pressure".
44. …the pressure created by the Owners in their demand for a waiver of rights by the Charterers has to be seen both in the light of their repudiatory breach and in the light of their subsequent conduct, including their deliberate refusal to comply with the assurances they had previously given about providing a substitute vessel and paying full compensation in respect of that breach. Their refusal to supply the substitute vessel to meet the Charterers' needs, in circumstances which they had created by their breach and their subsequent misleading activity, unless the Charterers waived their rights, could readily be found by the Arbitrators to amount to "illegitimate pressure".