Apple Mac Performa 640 series - Manual (Page 153)

3 Rotate the SIMM gently to the back until you feel it snap into place.
The plastic pin on the SIMM slot
should engage the hole in the SIMM.
Be sure that the metal clips snap into place.
Rotate the SIMM back until it snaps into place.
145 Installing an Expansion Card or Memory for the PC Environment