When installing the back wheel of a singlespeed, where the chain tension is adjusted only by the placement of the axle on the dropouts (such as a kids coaster-brake bike), you can use any blunt tool as a lever between the bottom bracket (as the fulcrum) and the wheel so that you can keep the wheel pushed out with one hand while you ratchet the axle nuts with the other.

or you could just hold the tire by the rim and pull lol