You can move money into your Rainy Day Fund by messaging 'Save' to Digit. You can also say things like "Save 10" to save $10 to your Rainy Day Fund. When within the 'Save' conversation, before you confirm the transfer, you can text 'Cancel' to change your mind.
Once you've initiated and confirmed a transfer into your Rainy Day Fund, this transaction can take up to 3 business days before the funds are fully posted and can be withdrawn back to your checking account. Often this is faster, but please be certain before confirming the transfer.