As a reminder, a unilateral contract is where an offeree accepts through performance. A common example that professors like to use is: A says to B, “If you walk across the Brooklyn Bridge, I’ll pay you $100.” To get paid, B must cross the bridge. Promising to cross the bridge is not enough.
Our traditional rule is that A can revoke this offer until B completes performance. So if B starts walking across the bridge, A can say, “I take back my offer!” Now B does not get paid even if she crosses the bridge because A revoked the offer.
The modern rule is different – – unilateral contracts cannot be revoked once performance begins. That is, if B starts performing, A cannot revoke the offer. In the above example, if B is crossing the bridge, A cannot revoke the offer.
Below is an older video which discusses terminating offers. Around the 3:50 mark the video discusses revoking unilateral contracts.