[exacube]

IIUC:

OP is set to own roughly 40% in 4yrs -- 10% at the end of the 1st year (i.e the "cliff"), and the rest of the 30% across the remaining 3 years (probably more often than once a year). the "reverse vesting" means that everytime OP vests, OP's cofounder has the right to buy off the newly vested shares. this means that OP would get money but lose ownership if their cofounder chose to bought off the shares.

OP has been working 11 months so far and is approaching the 1 year cliff (so set to own 10%), so now their cofounder wants OP to leave the company and take only 3% ownership, and is threatening to leave and abandon the company risking any value at all. Presumably, cofounder also has the right to fire OP though OP hasn't stated anything about this.