Taildragger123,
It is exceeding 0.9M that results in the uncontrollable pitch down; exceeding 300 KIAS when below 0.9M is fully controllable. The limit is always promulgated as M0.9/300kts but the reasons for the M and IAS limits are different. To put this into the context of display flying, please note that M0.9 at ISA, sea level is 595 kts.