I found this in the 777 FCOM 4.10.19. It seems to take the 744 logic a step further and will disarm LNAV and VNAV even on the ground, when above 80 knots. Maybe someone can explain the logic behind this. I'm thinking that for a windshear encounter during takeoff, you might not want LNAV or VNAV to go active?
Takeoff/Go-around (TO/GA) Switches
On the ground:
Push –
• below 50 knots and flaps out of up, activates autothrottle in THR REF mode at reference thrust limit selected on THRUST LIMIT page. If not pushed below 50 knots, autothrottle operation is inhibited until reaching 400 feet altitude
• pushing either switch above 80 knots disarms LNAV and VNAV
In flight:
Push (after lift-off with takeoff reference thrust limit displayed) –
• removes takeoff and climb derates and assumed temperature thrust reduction
• A/T in HOLD, activates A/T in THR REF mode
• disarms LNAV and VNAV, if armed
• between 50 feet and 400 feet, selects TO/GA roll mode
• above 400 feet, selects TO/GA roll and pitch modes